The standard rate constant (k⁰) and transfer coefficient (α), characterising the heterogeneous electron-transfer reaction, have been evaluated for glassy carbon electrodes modified with [Os(bipy)₂(PVP)_nCl]Cl (bipy = 2,2′-bipyridine, PVP = poly(4-vinylpyridine), 25 n 5) using sampled current voltammetry. The effect of variations in osmium content within the film, electrolyte concentration and temperature on these parameters is examined in hydrochloric and perchloric acid. The relevant enthalpies (ΔH⁰_#)_real, (ΔH⁰_#)_ideal and entropies (ΔS⁰_#)_real, (ΔS⁰_#)_ideal, as well as, the reaction entropy of activation (ΔS⁰_rc) for the heterogeneous electron-transfer reactions at the electrode/metallopolymer interface have been evaluated. The influence of the immobilised film on doublelayer formation and heterogeneous kinetics is discussed and the results correlated with homogeneous charge-transport data. The results obtained suggest that the changes in the supporting electrolyte and redox site loading influence the physico-chemical properties of the polymer phase rather than the local microenvironment of the redox centre.