The nature of the interaction of CO with NiO(100) thin films (approximately 20-50 angstrom) grown on a Mo(100) substrate has been investigated with infrared reflection absorption spectroscopy. Isosteric heats of adsorption were measured for CO in a pressure range of 1 X 10(-8) to 1 Torr and over a temperature range of 93-280 K. The average heat of adsorption was found to be approximately 10.4 kcal/mol, with the heat of adsorption decreasing slightly with increasing CO coverage. This value for die heat of adsorption agrees very well with that measured for CO on MgO(100). The CO stretching frequency was found to decrease with an increase in the CO coverage. At constant coverage, the CO stretching frequency as well as the full width at half-maximum were found to increase with increasing temperature. These results are discussed in the context of current theories of bonding between CO and oxide surfaces.