Stable films made from the ionomer poly (ester sulfonic acid) Eastman AQ55 on pyrolytic graphite (PG) electrodes gave reversible voltammetry for the incorporated protein hemoglobin (Hb). Cyclic voltammetry of Hb-AQ films showed a pair of well-defined, reversible peaks at about - 0.04 V versus SHE at pH 5.5. Compared to solutions, electron transfer between Hb and PG electrodes was greatly facilitated in the AQ films. Soret absorption band positions suggest that Hb retains a near native conformation in AQ films in the medium pH range. The formal potential of the Hb heme Fe(III)/Fe(II) couple in AQ films shifted linearly between pH 4 and 11 with a slope of - 52 mV pH (-1), suggesting that one proton transfer is coupled to each electron transfer in the electrochemical reaction. Square wave voltammograms of Hb-AQ films were fit by nonlinear regression analysis using a model featuring dispersion of formal potentials. Hb can act as an enzyme-like catalyst in these films as demonstrated by catalytic reduction of trichloroacetic acid with significant decreases in the electrode potential required.