What is the MST file format?
Files with the .mst extension, also known as transform files, are used by the Microsoft Windows operating system for storing Windows Installer (msiexec.exe) application installation settings. An .mst file contains installation configuration and parameters that control how a Windows application is installed, without altering the original installer package.
The purpose of .mst files
The main use of .mst transforms is to customize Windows Installer settings when installing various applications - pre-setting feature selections, suppressing dialog boxes, redirecting the install directory, or silently accepting license terms. The contents of an .mst file are updated each time the preferred installation settings for a given application are modified.
Where are .mst files used?
.mst files are commonly used by IT administrators and enterprise deployment teams to roll out standardized, pre-configured software to managed workstations. They are also used by programmers during software development and software version testing.
.mst files and MSI packages
.mst files work alongside a base .msi package. They allow users to change local installation settings without modifying the Windows Installer package itself. A transform is applied at install time using the TRANSFORMS property:
msiexec /i installpkg.msi TRANSFORMS=transforms.mst
Multiple transforms can be chained by separating file names with semicolons. Internally, .mst files are binary COM Structured Storage (OLE compound) databases - the same container format used by .msp patch files - storing only the delta between a reference installer database and the customized target state.
Security & safety
RISK: MEDIUMAn MST is data, not directly executable, but it modifies how an installer runs - a malicious transform could change install paths, disable security prompts or alter which components are installed. Only apply transforms from trusted sources (your IT department or the software vendor) alongside their intended MSI. Inspect an unfamiliar .mst in Orca before deploying it. Because installs often run with elevated/admin rights, a tampered transform deployed at scale is a real supply-chain concern - verify provenance.
Format details
in a nutshellPrograms that open MST files
Windows4 apps
Orca (Windows SDK MSI tools)
Free Open the base MSI in Orca, then Transform → Apply Transform to view, or Transform → New Transform → edit → Generate Transform to create/edit the .mst.
Windows Installer (msiexec, built-in)
Built-in Apply it to its MSI: 'msiexec /i package.msi TRANSFORMS=custom.mst /qn'. The MST is not run on its own.
InstEd (MSI/MST editor)
Freemium Open the MSI, then load/create the transform to inspect or edit changes - a friendlier alternative to Orca.
Advanced Installer
Freemium Use its MSI/transform tooling to author or apply an .mst for repackaging/deployment. Technical details
deep spec| Container format | COM Structured Storage (Compound File Binary Format / OLE2) - the same binary container used by .msi packages, .msp patches, and legacy Office documents |
| File signature (magic bytes) | D0 CF 11 E0 A1 B1 1A E1 at offset 0 - standard OLE2 header shared with other Compound File formats; not unique to .mst |
| MIME type | application/octet-stream - no dedicated registered MIME type exists for .mst |
| Developer | Microsoft |
| Content model | Stores only the delta - rows that differ between a reference MSI database and the customized target state; not a standalone full installer database |
| Applied via | TRANSFORMS= property on the msiexec command line, e.g. msiexec /i package.msi TRANSFORMS=custom.mst |
| Transform chaining | Multiple .mst files can be applied in sequence by separating their paths with semicolons in the TRANSFORMS value |
| Transform validation flags | Flags such as MSITRANSFORMVALIDATION_PRODUCT lock a transform to a specific product code, version, or language to prevent mis-application |
| Transform error conditions | Error condition flags such as MSITRANSFORMERROR_CHANGECODEPAGE define behavior when a transform is applied to a mismatched or incompatible package |
| Group Policy support | Can be specified alongside an .msi package in Group Policy Software Installation for automated pre-configured enterprise rollout |
| Platform | Windows only - Windows Installer is a Windows-exclusive technology; .mst files have no equivalent on macOS or Linux |
| Relation to .msp patches | .msp (Windows Installer Patch) uses the same OLE compound container but delivers binary file patches; .mst delivers table-level database changes only |
| Editing tools | Orca (Windows SDK) is the official free MST editor; InstEd and Advanced Installer also support authoring; msiexec.exe applies but cannot author transforms |
| Typical enterprise use | Pre-configuring MSI-packaged software (e.g., Office, Acrobat) for standardized deployment via SCCM, Microsoft Intune, or Group Policy |
| Released | Windows Installer (MSI) era, ~1999 (Office 2000 / Windows 2000) |
| Specification | learn.microsoft.com |
MST conversions
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