What is the ONETOC2 file format?
.onetoc2 is a binary metadata file automatically created and managed by Microsoft OneNote to index the contents of a local notebook folder. It is not a note file - it records which .one section files belong to the notebook, their display names, internal GUIDs and the order in which they appear in the OneNote navigation panel.
Introduced with OneNote 2010 to replace the older .onetoc format (used in OneNote 2003-2007), .onetoc2 uses the OneNote Revision Store binary format documented by Microsoft in the [MS-ONE] open specification. The file begins with a 16-byte GUID signature and a 1 KB header block containing version information and pointers to file node lists that index the notebook's sections.
OneNote regenerates a missing or corrupted .onetoc2 automatically. Users should not edit or delete it manually - doing so breaks notebook navigation until the file is rebuilt. The file grows modestly with the number of sections in the notebook and inherits optional password encryption if the notebook is protected.
On OneDrive and SharePoint, OneNote uses a different cloud-native storage format; .onetoc2 files are specific to locally stored notebooks and SharePoint-based notebook libraries.
Security & safety
RISK: LOW.onetoc2 files are auto-generated index metadata. They pose no security risk in normal use. If received in email, it is unusual and meaningless without the accompanying .one section files. Microsoft OneNote notebooks CAN contain embedded scripts or macros in .one sections, but the .onetoc2 file itself is purely structural metadata.
Format details
in a nutshellPrograms that open ONETOC2 files
Windows2 apps
Microsoft OneNote
Freemium Open the parent notebook folder in OneNote (File > Open Notebook), not the .onetoc2 file directly. OneNote reads it automatically. 
Microsoft Office
Paid OneNote is included in Microsoft 365 / Office suites. Open via OneNote application; the .onetoc2 file is read automatically when opening the notebook folder. 
macOS1 app
Microsoft OneNote (macOS)
Freemium Open the notebook folder via File > Open Notebook. OneNote for Mac handles .onetoc2 automatically. 
Android1 app
Microsoft OneNote (Android)
Freemium Open the synced notebook via OneDrive; the .onetoc2 file is managed automatically in the background.
OneNote Online (Microsoft 365)
Freemium For cloud-synced notebooks; .onetoc2 is managed server-side. Access notebooks via browser. Technical details
deep spec| Magic bytes | 16-byte GUID {7B5C52E4-D88C-4DA7-AEB1-5378D02996D3} at offset 0 |
| TOC GUID | {109ADD3F-911B-49F5-A5D0-1791EDC8AED8} in header (identifies the file as a TOC rather than a section) |
| Container | OneNote Revision Store binary format (Microsoft MS-ONE specification) |
| Encoding | Binary, little-endian |
| Stores | GUIDs, display names and display ordering of .one section files within the notebook folder |
| Compression | None (index data stored uncompressed) |
| Encryption | Optional; inherits notebook password protection if the notebook is password-protected |
| Typical size | 2 KB - 50 KB (grows modestly with number of notebook sections) |
| Integrity | CRC per file node block; revision history tracked in the revision store |
| Replaces | .onetoc legacy format (OneNote 2003-2007) |
| MIME type | application/msonenote |
| Behavior | Auto-created and rebuilt by OneNote; manual editing or deletion breaks notebook navigation |
| Released | 2010 (OneNote 2010 introduced .onetoc2; replaces legacy .onetoc) |
| Latest version | Revision Store format as documented in [MS-ONE] v20231129 |
| Specification | learn.microsoft.com |
ONETOC2 conversions
Community Q&A
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