The electropolymerization of manganese tetra(o-aminophenyl)porphyrin (MnTAPP) produces stable and electrochemically active films from an acetonitrile solution and from strong acidic aqueous solutions. These films can be produced on glassy carbon, gold, and transparent semiconductor tin oxide electrodes. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of poly(MnTAPP) films. MnTAPP is a molecule that has four branched monomers. The polymer films showed redox couples, and the cyclic voltammograms were pH dependent when transferred to various acidic aqueous solutions. The electrocatalytic reduction of SO 5 2 - and S 2 O 8 2 - by poly(MnTAPP) in strong acidic aqueous solutions showed obvious electrocatalytic reduction activity. The poly(MnTAPP) films also electrocatalytically oxidized ascorbic acid, SO 3 2 - , S 2 O 4 2 - , and Fe2+ in aqueous solution.