What is the MO file format?
.mo is a developer file used for storing a compiled binary translation catalog. .mo files are not human-readable - they are the machine-readable counterpart to a human-editable .po source file. Translators edit the .po file and compile it to .mo using the msgfmt command; the application then loads the .mo at runtime to display translated text. Modifying a .mo file directly will render it unusable.
Data access
.mo files are not encrypted or read-protected. Some hex editors can display the raw binary, but the correct way to recover human-readable strings is the msgunfmt command, which decompiles a .mo back into its .po source. Internally, a .mo starts with a four-byte magic number (0x950412DE little-endian), a revision word, and two offset tables pointing to original (msgid) and translated (msgstr) string arrays. An optional hash table enables fast runtime lookup.
Security & safety
RISK: LOWA .mo holds only translation strings and metadata - there is no executable code, so the file itself is harmless. The only practical caution: a malicious .mo placed in a theme/plugin could inject misleading interface text (e.g. altered links or instructions), so only use language files from the official theme/plugin source, not from unknown 'translation' downloads.
Format details
in a nutshell- Autodesk Maya ASCII/object data - Some Maya pipelines and exporters use .mo for object data - unrelated to gettext.
- Modula-3 object file - Historic compiled object file for the Modula-3 language; the DB name reflects this rare meaning.
- Motorola / firmware object dumps - Occasionally seen for Motorola object/firmware blobs; niche and unrelated.
Programs that open MO files
Windows2 apps
Poedit
Freemium File > Open the .mo to view it, or open the matching .po, translate, and save - Poedit writes the .mo automatically.
GNU gettext (msgunfmt / msgfmt)
Open-source Run 'msgunfmt file.mo' to print the catalog as .po text, or 'msgfmt file.po -o file.mo' to compile. 
macOS2 apps
GNU gettext (Homebrew)
Open-source brew install gettext, then 'msgunfmt file.mo' / 'msgfmt file.po -o file.mo'. 
Linux3 apps
GNU gettext
Open-source Preinstalled on most distros: 'msgunfmt file.mo' to read it, 'msgfmt' to compile.
Poedit (web service for catalogs)
Freemium Use the desktop app; .mo files are best handled locally rather than via random online viewers. Technical details
deep spec| Format type | Binary compiled translation catalog (hash-indexed string table) |
| Magic number | 0x950412DE (little-endian: DE 12 04 95 at offset 0; big-endian files store 95 04 12 DE) |
| Byte order | Little-endian or big-endian; reader detects byte order from the magic number at parse time |
| MIME type | application/x-gettext-translation |
| String encoding | UTF-8 (most common); actual charset declared in the empty-msgid metadata entry |
| Metadata entry | Entry with empty msgid stores Project-Id-Version, Content-Type, charset, and Plural-Forms rules |
| Format revision | Revision 0 (both major and minor revision words are 0); revision 1 is defined but rarely deployed |
| Hash table | Optional fixed-size hash table embedded in the file body for O(1) string lookup at runtime |
| Compilation tool | msgfmt (GNU gettext) compiles .po → .mo |
| Decompilation tool | msgunfmt (GNU gettext) recovers a .po file from a .mo binary |
| Typical file size | 5 KB - 2 MB depending on the number of translated strings |
| Platform support | Linux, macOS, Windows, BSD - any OS running gettext-based software |
| Common deployment path | Installed as <domain>.mo inside an LC_MESSAGES directory under a locale tree (e.g. locale/fr/LC_MESSAGES/app.mo) |
| Released | 1995 (GNU gettext) |
| Latest version | MO format revision 0 (still current); revision 1 defined but rarely used |
| Open standard | Yes · royalty-free |
| Specification | www.gnu.org |
MO conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about MO files.
