Mechanisms for the pyrolysis of formic acid in the gas-phase catalyzed by water dimer or formic acid itself are proposed for the first time. At the B3LYP/6-311 ++G(3df,3pd)//B3LYP/6-311++G(d,D) level, the barrier heights for both dehydration and decarboxylation reactions are revealed to be significantly lower than previously reported values, implying the importance of the catalytic effect of (H2O)(2) and HCOON.