Infrared reflection absorption spectroscopy (IRAS) has been used to study CO adsorption on Au clusters ranging in size from 1.8 to 3.1 nm, supported on TiO2. The adsorbed CO vibrational frequency blue-shifts slightly (approximately 4 cm(-1)) compared to that adsorbed on bulk Au, whereas the heats of adsorption (-DeltaH(ads)) increase sharply with decreasing cluster size, from 12.5 to 18.3 kcal/mol.