Two types film of tin silicomolybdate and tin oxide/silicomolybdate films have been prepared directly from the mixing of Sn2+ and SiMo12O404- ions from the strong acidic solutions over different scanning potential ranges. The cyclic voltammogram of the growing type I film (assigned as a tin silicomolybdate film) is characterized by two redox couples and the type II film (assigned as a tin oxide/silicomolybdate film) exhibits three redox couples and both films were confined to the immobilized film when transferred to a pH 1.0 aqueous solution. In addition to cyclic voltammetry, an electrochemical quartz crystal microbalance and UV-visible absorption spectroscopy were used to study the growth mechanism of the tin silicomolybdate and tin oxide/ silicomolybdate films. The results show that Sn2+ and SiMo12O404- ions react slowly to form a new blue-colored species with absorption peaks at gimel = 320, 645, and 744 nm. The electrocatalytic reduction of ClO3-, BrO3-, IO3-, SO52-, S2O82-, and NO2- ions by a tin oxide/ silicomolybdate film or a tin silicomolybdate film were investigated in acidic aqueous solutions. The electrocatalytic reactions of IO3- ions with a tin oxide/ silicomolybdate film were investigated using the rotating ring-disk electrode method. (c) 2005 The Electrochemical Society.