The mechanism of the decomposition of formic acid HCOOH on ZnO(<10(1)over bar 0>) surfaces was investigated using the ab initio molecular orbital method. Furthermore, the role of the dynamic bending of the surface formate anion HCOO- in the decomposition reaction was also examined. The lattice Zn atom interacts with the C-H bond of the adsorbed formate anion and cleaves the C-H bond to yield adsorbed CO2 and ZnH species. However, similar results are not obtained with the lattice O atom or surface OH species. The energy barriers of C-H bond cleavage and CO2 desorption from the surface were calculated to be 45.5 and 11.2 kcal/mol, respectively, at the MP2 level. H-2 formation is a bimolecular process : another formic acid from the gas phase attacks the ZnH surface species and produces an H-2 molecule. The calculated energy barrier is 2.2 kcal/mol and the exothermicity of the reaction is 11.3 kcal/mol at the MP2 level. These results indicate that the rate-determining step of the dehydrogenation decomposition of formic acid is cleavage of the C-H bond. Both the decomposition reaction and the dissociative adsorption of HCOOH occur more easily on hydrogen-covered surface than on a clean surface.