Adsorption states and photoreactions of CO and D2O adsorbed on NiAl(110) at 90K were studied by temperature-programmed desorption (TPD). Coverage dependence of TPD spectra showed growth behavior of adlayers of these molecules consistent with the literature, but a new path of thermal dissociation of D2O upon adsorption at 90K was found. The photoreaction cross sections of CO and D2O at 4.7eV were estimated in the order of 10−21 cm2 and 10−20 cm2, respectively. This result suggests that photoexcitation of the NiAl(110) substrate itself, a typical substrate used in supported model catalyst studies, will induce only negligible photoreactions of molecules adsorbed at surfaces of metal nanoparticles supported on ultrathin alumina films formed on NiAl(110).