ISZ File Documentation

Summary

An ISZ (Zipped ISO Disk Image) file is a compressed, and optionally encrypted or split, ISO disc image created by EZB Systems’ UltraISO. Think of it as a zipped ISO: the same optical-disc image, shrunk with zlib or bzip2. It begins with the signature IsZ!. The simplest path is to convert it back to a plain .iso and mount that, since Windows 8/10/11 mount ISO natively. UltraISO, the free isz-tool, AnyToISO and DAEMON Tools all handle ISZ.

Technical details

FeatureValue
Full nameZipped ISO (UltraISO Compressed Disc Image)
File extension.isz
MIME typeapplication/x-isz
Format typeCompressed (optionally encrypted/split) optical-disc image
DeveloperEZB Systems (UltraISO)
IntroducedMid-2000s
Open standardNo — proprietary
Base imageISO 9660 / UDF disc image
Compressionzlib or bzip2 (block-based)
EncryptionOptional AES, password-protected
SplittingOptional segments (.isz, .i00, .i01 …)
Magic number (hex)49 73 5A 21 (ASCII IsZ!) at offset 0
Native appUltraISO
Free conversionisz-tool, AnyToISO (free edition)
Direct mountDAEMON Tools Lite
Related extensions.iso, .daa, .uif, .mdf, .img, .nrg
Specificationezbsystems.com/isz/index.htm
File signature (magic bytes)
49 73 5A 21

Offset 0, 4 bytes. In ASCII this reads IsZ!, the ISZ header marker. The header that follows records the compression method (zlib or bzip2), whether the image is AES-encrypted, and any segment/split information for images spread across .i00, .i01 parts. Once those layers are removed, the payload is an ordinary ISO 9660/UDF disc image.

What is an ISZ file?

ISZ is short for “zipped ISO”. It is EZB Systems’ proprietary disc-image format, introduced in the mid-2000s for their UltraISO software. An ISZ is a standard ISO 9660/UDF optical-disc image that has been compressed to save space, and it can additionally be AES-encrypted with a password and split into several segments so a large image spans multiple files. The format begins with the ASCII signature IsZ!. The relationship is simple: ISZ is to ISO what a ZIP archive is to a folder, with optional encryption and splitting layered on top.

You usually meet an ISZ when you download software, a game or an operating-system image that someone packaged with UltraISO to make it smaller, or when you compressed your own disc backups. Because the format is proprietary and Windows’ built-in ISO mounting does not understand it, the practical approach is almost always to convert it back to a plain ISO, which every operating system and tool supports.

The IsZ! header and its three layers

An ISZ file starts with a header identified by the 4-byte IsZ! signature at offset 0. The header describes how the original disc image was transformed before storage, and understanding those transformations explains everything about how you open the file. There are three optional layers applied over the base disc image:

LayerWhat it doesWhat you need to reverse it
CompressionShrinks the image with zlib or bzip2, block by blockAny ISZ-aware tool (decompresses automatically)
EncryptionAES-encrypts the contents behind a passwordThe correct password — no way around it
SplittingDivides the image into segments across .i00, .i01 …All parts, kept together in one folder

Compression is the layer every ISZ has: the disc data is divided into blocks and each is compressed with zlib or bzip2, which is what makes the file smaller than the ISO it came from. Encryption is optional; when present, the image contents are protected with AES and the password is mandatory to decompress them. There is no workaround for a lost password, because the compressed data itself is encrypted. Splitting is also optional and is used to fit a large image onto smaller storage or across a size limit; the pieces are named .isz for the first segment and .i00, .i01 and so on for the rest.

The payload: a standard ISO underneath

Once the compression (and any encryption) is reversed and the segments are rejoined, what remains is an ordinary ISO 9660 or UDF disc image, the same universal format every burning and mounting tool reads. Nothing about the disc’s contents is changed by the ISZ wrapper; the files, boot sectors and directory structure are identical to the source disc. This is why converting ISZ to ISO is not really a “conversion” in the sense of re-encoding: it is decompression back to the image that was compressed in the first place.

Converting an ISZ to ISO

Because ISO is universal and Windows 8, 10 and 11 mount it natively (double-click, or right-click › Mount), the key task is almost always to produce a plain ISO. Several tools do this:

UltraISO      File > Open the .isz, then Tools > Convert > ISO
isz-tool      open-source command line: converts .isz to .iso
AnyToISO      free edition: Open the .isz, Convert to ISO
PowerISO      Open the ISZ to browse, extract, mount or convert

UltraISO is the format’s own creator (paid trialware) and can also mount an ISZ directly to a virtual drive. For a free route, the open-source isz-tool and AnyToISO’s free edition both decompress an ISZ into a plain ISO, and DAEMON Tools Lite can mount an ISZ directly without converting it first. A password-protected ISZ requires the password before it will decompress, and a split ISZ needs all its parts in the same folder; opening the first .isz rejoins them automatically.

Why converting an important ISZ matters

ISZ is a legacy, niche format today, as optical media and UltraISO’s prominence have both faded. The container is just compressed disc data, so it carries no special risk on its own, but the disc it images can contain anything, including an autorun installer on a pirated image, so scan extracted contents and only download OS or software images from official sources. The more practical long-term risk is obsolescence: fewer tools support ISZ each year. Converting an important ISZ to the open ISO format now means you can still open it years from now, regardless of whether UltraISO is still around.

Frequently asked questions

How do I open an ISZ file?

Convert it to a plain .iso with UltraISO, the free isz-tool, AnyToISO or PowerISO, then mount the ISO. Windows 8/10/11 mount ISO by right-click › Mount. Alternatively, mount the ISZ directly with DAEMON Tools Lite, which understands the format.

What is the difference between ISZ and ISO?

ISZ is a compressed, and optionally encrypted or split, ISO made by UltraISO; think of it as a zipped ISO. ISO is the open, universally supported disc image that Windows mounts natively. For compatibility, convert ISZ to ISO.

My ISZ is password-protected, how do I open it?

ISZ supports AES encryption, so you must enter the correct password in UltraISO (or another tool that supports encrypted ISZ) to decompress it. Without the password the image cannot be opened; there is no workaround.

Why is my ISZ split into .i00, .i01 files?

It was saved as a split ISZ to fit smaller storage. Keep all the parts (.isz plus .i00, .i01 …) in one folder and open the first .isz; the tool rejoins them automatically when converting or mounting.

References