Stable electroactive mixed films of osmium oxide/hexachloroplatinate, osmium oxide and platinum have been deposited on different electrode materials by cycling the electrode potential repetitively in solution containing Os 3þ and PtCl 2À 6 . The film growth of was monitored by using cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). The cyclic voltammetric features of modified electrode in Os 3þ solution resembles that of surface wave, and involves ions-exchange with Os 3þ ions present in the solution. Based on the SEM results the modifier was considered as one-dimensional, mixed-valent polymeric film, and stabilized by Os 3þ/2þ counter ions. Finally, the electrocatalytic activity of the modified electrode was examined toward reduction of NAD þ , chloroacetic acids and nitrous oxide.