Gas-phase decomposition of formic acid results in final products CO + H2O and CO2 + H-2. Experimentally, the CO/CO2 ratio tends to be large, in contradiction with mechanism studies, which show almost equal activation energies for dehydration and decarboxylation. In this work, the influence of H-2 on the decomposition mechanism of HCOOH was explored using ab initio calculations at the CCSD(T)/6-311++G**//MP2/6-311++G** level. It was found that, in the presence of H2, the reaction channels leading to CO + H2O are more than those leading to CO2 + H-2. With competitive energy, H-2 addition to HCOOH can reduce the latter into HCHO, which then dissociates into CO + H-2 catalyzed by H2O. Compared to trans-HCOOH, cis-HCOOH and cis-C(OH)(2), conformers required for decarboxylation, are less populated due to interactions with H-2.