The adsorption and reaction of CO and HCOOH on the NiO(100)/Mo(100) and NiO(111)/Mo(110) surfaces have been studied using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). Significant differences have been found for the two faces of NiO regarding the adsorption and reaction of HCOOH. While molecularly adsorbed formic acid is stable up to 200 K on the NiO(100) surface, formic acid decomposition to formate occurs on the NiO(111) surface at 100 K. Upon heating to 700 K, most of the formate on the NiO(111) surface dehydrogenates or dehydrates, while similar to 70% of the formate species on the NiO(100) surface desorbs as molecular formic acid. With respect to CO adsorption, the NiO(111) surface shows a slightly higher binding energy than does the NiO(100) surface.