The photochemistry of carbonyl sulfide (OCS) adsorbed to small silver clusters is shown to exhibit a striking odd-even dependence on the number of Ag atoms in the cluster. OCS is found to desorb nondissociatively from even numbered silver clusters. In contrast, on odd silver clusters, a new product channel corresponding to AgnS is observed. Parallels are found with the photochemistry of adsorbates on extended surfaces. Cross-section measurements for OCS desorption from Ag-10 and wavelength-dependent measurements of OCS dissociation on Ag-9 both indicate that electronic excitation of the cluster initiates chemistry, analogous to substrate mediated surface photochemistry. The size dependence is reasonably explained in terms of a charge-transfer mechanism involving an ion-pair state of the AgnOCS complex.