Tin nitride thin films were prepared by radio-frequency (RF) reactive sputtering. The metallic tin target was sputtered by nitrogen gas with RF magnetron sputtering equipment. Transparent polycrystalline SnNx thin films were obtained at an RF power of 50 W. On application of -3.0 V in a 1.0 MNa(2)SO(4) solution, SnNx decomposed into Sn through a reaction with a proton. On subsequent application of +3.0 V, Sn dissolved into the aqueous solution, until inactive SnO2 was formed at the surface of the Sn. These irreversible changes were accompanied by an increase and a decrease in optical absorption.