The decomposition of formic acid on (2 x 2) NiO(111)/Ni(111) was studied by infrared reflection absorption spectroscopy (IRAS). Formic acid molecules adsorbed dissociatively on the surface to form formate which decomposed to H-2, CO2, H2O, and CO at 340-415 and 520 K. The vacant sites after the decomposition of formate were probed by the adsorption of CO at 100 K. On clean NiO(111) at 100 K, IRAS peaks of two types of adsorbed CO were observed at 2146 and 2079 cm(-1), which were assigned to the CO on fully-oxidized Ni cation sites and less-oxidized Ni cation sites, respectively. The CO peaks were not observed on the formate precovered surface. The IRAS peak of adsorbed CO at 100 K on the fully-oxidized Ni cation sites appeared when the surface was heated to decompose formate around 340-415 K. After decomposition at 520 K, the intensities of both peaks of CO were recovered to the initial intensities. It was thus concluded that the sites of decompositions at 340-415 and 520 K are the fully-oxidized Ni cation sites and less-oxidized Ni cation sites, respectively. The behavior of hydroxyl species produced by dissociative adsorption of formic acid was next investigated using deuterated formic acid (DCOOD). The hydroxyl species (-OD) formed by the dissociative adsorption of formic acid (DCOOD) was observed at 2536 cm(-1) below 343 K, while the isolated OD groups appeared at 2713 cm(-1) and around 500 K. The mechanism of decomposition of formate at each temperature is discussed.