The plasmon absorption band of 6-nm silver particles in aqueous solution is blue-shifted and strongly broadened upon the adsorption of phosphine, PH3. The effects are more pronounced in alkaline than acidic solution. In alkaline solution, the adsorption is completely irreversible. Upon aging of the silver solutions containing PH3, hydrogen and phosphate are generated in the mole ratio 4:1. A mechanism is proposed for the catalytic action of the silver particles in the reaction of phosphine with water, in which donation of electron density from absorbed phosphine into the silver particles is an important step. Conductivity measurements indicate that phosphine in alkaline solution is adsorbed as PH2-. It is shown in coadsorption experiments that phosphine is more strongly adsorbed than I- and that SH- and phosphine are about equally strongly adsorbed.