Tin hexachloroplatinate (SnPtCl6) films have been prepared by mixing Sn2+ and PtCl6 2− ions in an aqueous KCl solution. An electrochemical quartz crystal microbalance (EQCM), a rotating ring-disk electrode, UV–Vis absorption spectroscopy, and cyclic voltammetry were used to study the deposition and growth mechanism of the tin hexachloroplatinate films. The electrochemical properties of the films indicate that the redox processes were confined to the immobilized tin hexachloroplatinate films. Deposition of a tin hexachloroplatinate film occurs when PtIVCl6 2− is electrochemically reduced to PtIICl6 4−, or when Sn2+ reacts with PtIVCl6 2− and PtIICl6 4− ions in an aqueous KCl solution. The electrocatalytic reduction of NAD+ and of sulfur oxoanions (S4O6 2−, S2O8 2−, and SO5 2−) was studied using tin hexachloroplatinate films. The electrochemical reactions of NAD+ and NADH were investigated using cyclic voltammetry and the rotating ring-disk electrode method.