What is the ISZ file format?
An .isz file is a compressed disk image file created by EZB Systems. The format is essentially a compressed .ISO image - a proprietary binary container designed to reduce the size of optical disc images while preserving their full content.
.isz follows closely the ISO format standard, using compression to achieve smaller file sizes that help save disk space. It supports two lossless compression methods - ZLIB and BZIP2 - which retain all file metadata. Each .isz file begins with the 4-byte ASCII header signature IsZ! (hex 49 73 5A 21) at byte offset 0, identifying it as an ISZ container rather than a plain .iso.
The format also offers password protection and encryption: AES-128, AES-192, or AES-256 (128-bit, 192-bit, and 256-bit key encryption, respectively).
Large .isz files can be split into smaller parts. Each part retains the name of the original file, but starting from the second segment the extension differs - the first part carries the .isz extension, the second .i01, the third .i02, and so forth. Splitting files into smaller fragments allows users to store data across multiple storage devices and reassemble the parts into a complete image when needed.
Similar proprietary compressed image formats include .DAA (PowerISO), .NRG (Nero), and .MDF (Alcohol 120%). Unlike those and the plain ISO standard, .isz is not an open format.
Security & safety
RISK: MEDIUMThe ISZ container is just compressed disc data, but the disc it images can contain anything - including an autorun installer or malware on a pirated game/software image - so scan extracted contents before running them and download OS/software images only from official sources. Because ISZ is proprietary and increasingly unsupported, the bigger practical risk is obsolescence: convert an important ISZ to the open ISO format so you can still open it years from now. Avoid download portals bundling 'ISZ openers' with adware; prefer the official UltraISO/AnyToISO sites or the open-source isz-tool.
Format details
in a nutshellPrograms that open ISZ files
Windows5 apps
UltraISO
Paid File > Open the .isz, then Tools > Convert to .ISO (or Mount to Virtual Drive). The format's own creator; paid trialware. 
AnyToISO
Freemium Open the .isz and Convert to ISO; the free edition handles images up to a size limit. 
isz-tool
Open-source Free command line: run it to convert .isz to .iso, then mount the ISO by right-click > Mount.
DAEMON Tools Lite
Freemium Mount the .isz directly to a virtual drive so Windows treats it like an inserted disc. 
macOS1 app
isz-tool (via terminal)
Open-source Build/run the command-line tool to convert ISZ to ISO, then double-click the ISO to mount. 
Linux1 app
isz-tool
Open-source Compile and run to convert .isz to .iso, then mount with 'mount -o loop image.iso /mnt'. Technical details
deep spec| Developer | EZB Systems |
| MIME type | application/x-isz |
| Magic bytes | "IsZ!" (hex 49 73 5A 21) at byte offset 0 |
| Compression methods | ZLIB and BZIP2 (both lossless) |
| Encryption | AES-128, AES-192, and AES-256 (optional, password-protected) |
| File splitting | Multi-segment archives: .isz (first part), .i01 (second), .i02 (third), and so on |
| Base format | Structurally derived from ISO 9660 optical disc image standard |
| File encoding | Binary (proprietary container) |
| Primary application | UltraISO (Windows) by EZB Systems |
| Platform support | Windows (primary); macOS and Linux via third-party tools such as isz-tool |
| Metadata retention | Full ISO metadata preserved through lossless compression |
| Virtual mounting | Can be mounted as a virtual optical drive in UltraISO and DAEMON Tools Lite without prior extraction |
| Header structure | Fixed-length binary header after 4-byte signature; encodes compression type, encryption flag, and segment info |
| Released | Mid-2000s, by EZB Systems for UltraISO |
| Specification | www.ezbsystems.com |
ISZ conversions
Community Q&A
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