The wavelength dependent photochemical cross sections for three adsorbates (OCS, NO, SO2) on roughened silver have been measured, and contrasted with the behavior on Ag(111). Surface roughness leads to significant enhancements of the photochemical cross sections for all three adsorbates. The enhancement exhibits a maximum at 350 +/-5 nm. Competing enhancement mechanisms are considered. Temperature programmed desorption measurements show that new adsorption sites are available on the surface, but that these are not uniquely associated with the enhanced cross section. The coincidence of the peak enhancement for both photodissociation of OCS and photodesorption of NO and SO2 suggests a substrate mediated mechanism. It is proposed that the enhancement arises from surface plasmon excitation on the roughened surface. This mechanism may contribute to an enhanced cross section in two ways. First the collective surface plasmon excitation can decay to single particle, hot electron, excitations. The hot electrons so generated may attach to the adsorbates, to cause the photochemistry observed. Secondly, the enhanced local electric field at the surface can generate electron-hole pair excitations, which may also attach to the adsorbate.