What is the TMX file format?
.tmx (Translation Memory eXchange) is an open XML format for exchanging translation memories between computer-assisted translation (CAT) tools. A translation memory stores previously translated sentence pairs - source language alongside target language - and .tmx provides a vendor-neutral way to transfer that data between tools such as Trados Studio, memoQ, OmegaT, and Phrase.
The format was created in 1997 by the OSCAR working group of LISA (Localization Industry Standards Association) and reached its current stable revision as TMX 1.4b in 2005. A TMX 2.0 draft was started in 2007 but never finalized. After LISA declared insolvency in 2011, the specification moved to a Creative Commons license. Despite its age, TMX 1.4b remains the universal exchange standard accepted by every major CAT platform.
A .tmx file is a standard XML document. The root <tmx> element contains a <header> block carrying metadata (source language, creation tool) and a <body> holding translation units (<tu>), each pairing source and target segments via <tuv xml:lang> and <seg> elements. Files range from a few kilobytes to hundreds of megabytes for enterprise memories with millions of segments.
Related localization formats include .xliff for active translation project files and .tbx for terminology exchange.
Security & safety
RISK: LOWTMX is a plain XML text file; no executable code, no macros. Standard XML parser caution: extremely large files (>100 MB) can cause out-of-memory errors in small tools. No known exploits.
Format details
in a nutshellPrograms that open TMX files
Windows6 apps
OmegaT
Open-source Project → Import Translation Memory → select .tmx; segments available for reuse immediately. 
TMXEditor (Maxprograms)
Open-source File → Open → .tmx; cross-platform Java app for viewing/editing TMX. 
macOS3 apps
Linux2 apps
Technical details
deep spec| Encoding | Text XML; UTF-8 (required by spec); UTF-16 with BOM also valid per XML rules |
| Container | Plain XML document - no binary wrapper, no compression |
| Root element | <tmx version='1.4'> (version attribute identifies spec level; older tools emit 1.1 or 1.2) |
| Document structure | <tmx> → <header> (metadata) + <body> → <tu> (translation units) → <tuv xml:lang> → <seg> (text) |
| MIME type | application/x-tmx+xml (also text/xml) |
| Typical file size | 10 KB - 500 MB (small project TM to enterprise multi-million-segment memories) |
| Language identification | BCP 47 language tags on xml:lang attributes (e.g., en-US, fr-FR) |
| Inline markup elements | <bpt>, <ept>, <ph>, <it>, <ut> elements encode formatting codes and placeholders within segments |
| Governing body history | Created by OSCAR/LISA (1997); spec moved to Creative Commons after LISA insolvency (2011) |
| Platform support | Windows, macOS, Linux, web-based CAT platforms - universally accepted |
| TMX 2.0 status | 2007 working draft; never finalized - TMX 1.4b remains the de facto current standard |
| File signature | None (plain XML); identified by XML declaration and <tmx> root element |
| Released | 1997 (TMX 1.0 by OSCAR/LISA) |
| Latest version | TMX 1.4b (2005) - de facto current; TMX 2.0 working draft 2007 never finalized |
| Open standard | Yes · royalty-free |
| Specification | www.ttt.org |
TMX conversions
Community Q&A
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