A model combining Marcus theory with Gaussian distributions of kinetic and thermodynamic parameters was developed for square-wave voltammetry (SWV) and applied to the protein myoglobin in thin films of didodecyldimethylammonium bromide on electrodes. Nonlinear regression analysis allowed direct estimation of electron transfer rate constants and reorganization energies from single experiments at relatively-large pulse heights; Successful fitting relies on the accurate representation of background current within the model. Analysis of 22 voltammograms at pH 6 over a range of frequencies and pulse heights gave-a formal potential consistent with earlier determinations, and a mean log(k(RED)(null)) of 3.3 + 0.8 s(-1), not significantly different from the mean log(k(OX)(null)) of 3.0 +/- 0.6 s(-1). The mean value of reorganization energy lambda (RED) was 0.41 +/- 0.02 eV and lambda (OX) was 0.21 +/-0.01 eV. Differences in lambda (RED) and lambda (OX) are likely to reflect different solvation, bonding, and conformation near the iron heme regions of MbFe(III) and MbFe(II). Methods presented here promise to be generally applicable to determining Marcus electron-transfer parameters for redox proteins in thin films.