Detailed studies of the primary photochemical mechanism of two quite different adsorbates, OCS and NO, on a common substrate, AE(III), have been made. Irradiation of OCS on Ag(lll) at wavelengths shorter than 500 nm leads to dissociation into COg and S-a. For a Ag(111) surface saturated with NO, irradiation at all wavelengths studied results in desorption of both NO and N2O, The relative photochemical cross sections for the reactions have been measured as a function of the wavelength, polarization, and angle of incidence of the radiation. The polarization dependence was measured at four wavelengths and three angles of incidence, and the results were compared with predictions for both adsorbate and substrate localized excitation mechanisms. The results are similar for both adsorbates and consistent with a substrate excited mechanism. No evidence was found for a change in photochemical mechanism with wavelength for either adsorbate. The wavelength dependence of the photochemical cross sections was studied in greater detail, between 280 and 600 nm. For irradiation at wavelengths shorter than 400 nm, the wavelength dependence is essentially identical for each surface, strongly suggestive of a substrate excited mechanism. For the photodissociation, of OCS a threshold at ca. 500 nm was observed, while for NO desorption. no threshold was detected out to 600 nm. The wavelength-dependent cross sections were modeled assuming the primary photochemical mechanism to be attachment of hot, sub-vacuum electrons generated by substrate absorption. For OCS, the cross section was modeled assuming an adsorbate attachment level at 3.2 eV and a nascent distribution of hot electrons. For NO, at least two adsorbate attachment levels are required to reproduce the data and the effect of secondary hot electrons must be taken into account. It was concluded that all measurements could be interpreted on the basis of the substrate-mediated hot electron attachment mechanism, but that the limited knowledge of adsorbate electronic structure hampered more definitive conclusions.