Orthoformic acid or methanetriol is a chemical compound with the formula HC(OH)3.

In this molecule, the central carbon atom is bound to one hydrogen and three hydroxyl groups.

Orthoformic acid was long held to be a hypothetical chemical compound, as it was expected to decompose instantly into formic acid and water, making it too unstable to isolate or observe.

[2] However, observation was shown to be possible in 2024, when it was identified by mass spectrometry.

This involved the electron-irradiation of a frozen mixture of methanol and oxygen.