Calculations at both the SCF and MP2 levels including electron correlation effects on both the geometries and energies of different stationary points encountered along the decomposition pathways for thioformic acids are reported. Four conformers of monothioformic acid and two conformers of dithioformic acids have been investigated using ab initio electronic structure calculations with a series of basis sets. Results show that the keto tautomer of monothioformic acid, HC(:O)SH is the more stable form with respect to the thioketo tautomer, HC(:S)OH, Furthermore, all of the trans isomers of monothioformic and dithioformic acids are slightly more stable in energy,oy than the corresponding cis isomers, which are in agreement with experimental results, Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the unimolecular rate constants for pyrolysis of these acids are carried out.