The thermal decomposition of formic acid in the gas phase by a bimolecular self-reaction and by two H2O catalysed processes has been investigated by ab initio molecular orbital calculations using a modified Gaussian 2 (G2M) method, which has been tested for the present system with calculations for the geometry and stability of the dimer, (HCOOH)2, formed by the hydrogen bonding of two Z conformers. The dissociation energy for the dimer was calculated to be 13·9 kcalmol, which agrees closely with the experimental values, 12–15 kcal mol-1, and the result of a high-level ab initio calculation. The energy barrier for the self-reaction involving two Z conformers via transition state 1, producing 2H2O + 2CO, was found to be 67·9 kcal mol-1. The value is much greater than those for the H 2 catalysed decomposition of the Z conformer via transition state 2, producing H2O + CO + H2O, 48·2 kcal mol-1, and the H2O catalysed decomposition of the E conformer via transition state 3, producing H2 + CO2 + H2O, 44·2 kcal mol-1. Thermal rate constants for these processes, covering the 200–1000K temperature range, have been calculated with and without tunnelling corrections.