What is the TDT file format?
.tdt files contain THOR Data Tree records - the exchange format of Daylight Chemical Information Systems' THOR chemical database. .tdt files are used for exchanging chemical information in standardized form between programs that use different languages or syntax, making TDT a multiplatform data-sharing format.
.tdt files save data in plain text. However, their custom *datatree* syntax makes the contents virtually unreadable for the average person without a proper converter. Each record is anchored by a root SMILES structure string, followed by tagged dataitems (properties, names, cross-references) terminated by a | character - linking many properties to one compound in the "thesaurus" style that THOR is named for.
TDT has been largely superseded in general cheminformatics by SDF and flat SMILES files, which have broader tool support. Loading and querying .tdt files requires Daylight's THOR-Merlin software or Toolkit; a text editor shows the raw tagged structure but cannot interpret it without additional processing.
Security & safety
RISK: LOWA .tdt is passive text data, not executable, so it is low risk to open. The realistic concerns are practical: it's a niche Daylight-specific format, so without THOR/Merlin or the Daylight Toolkit you can only read the raw text, not work with the chemistry; and chemical datasets may be proprietary/confidential, so handle exported THOR data per your organization's data policy.
Format details
in a nutshell- Tab-Delimited Text data export - Some applications use .tdt as a generic tab-delimited text data file; open in Excel or a text editor - unrelated to Daylight THOR.
- Application-specific data/template files - A few programs reuse .tdt for their own data or template files; check the producing application.
Programs that open TDT files
Windows2 apps
Daylight Toolkit / THOR-Merlin
Paid Load/query the .tdt with Daylight's THOR-Merlin database tools (the native, intended way).
Notepad++ (read raw)
Open-source Open the .tdt to inspect the raw datatree text (SMILES + tagged fields); it won't interpret the chemistry. 
macOS1 app
Any text editor (read raw)
Free Open the .tdt as text to view the records; use Daylight tools for actual loading/querying. 
Linux1 app
Daylight Toolkit + text tools
Paid Use the Daylight Toolkit/THOR on Unix/Linux to load the datatree; inspect raw text with less/vim. Technical details
deep spec| Format type | Text-based chemical-information record (THOR datatree syntax) |
| Developer | Daylight Chemical Information Systems, Inc. |
| Encoding | ASCII plain text |
| Compression | None (THOR database uses indexed storage; the TDT exchange form is uncompressed text) |
| Typical size | KB to many MB (depends on the number of compound records) |
| Associated OS | Unix, Linux, Windows (with Daylight Toolkit / THOR-Merlin) |
| Record structure | SMILES root identifier + tagged dataitems (properties, names, cross-refs) + | record terminator |
| SMILES root | Each datatree is keyed by a SMILES or SMIRKS string identifying the compound by structure |
| Datatree concept | Multiple properties (thesaurus entries) attached to one compound root form THOR's searchable database |
| MIME type | text/plain (no registered MIME) |
| Tool requirement | Daylight Toolkit or THOR-Merlin required for loading and querying; text editors show raw content only |
| Current status | Largely superseded by SDF/MOL and flat SMILES files in general cheminformatics; remains at THOR-Merlin sites |
| Specification | Defined by the Daylight datatree specification at daylight.com |
| Released | With Daylight's THOR/Merlin chemical database system (1990s) |
| Latest version | Tied to the Daylight Toolkit / THOR-Merlin release |
| Specification | www.daylight.com |
TDT conversions
Community Q&A
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