The preparation of two copolymers of vinylferrocene and 4-(vinylpyridine) containing a few per cent of ferrocene groups and having molar masses of 2.3 X 10(5) and 2.4 X 10(4) gmol(-1) is described. The copolymers were dissolved in aqueous hydrochloric acid, where their electro-oxidation was examined by voltammetry at rotating disk electrodes, Severe adsorption on the electrode surface of the higher molar mass copolymer interfered with the evaluation of its diffusion coefficient, but values were obtained for the lower molar mass copolymer. The effects of pH, ionic strength and electrostatic binding of multiply charged counterions on the diffusion of both copolymers were examined. The uncertainties inherent in the use of electrochemical methods to evaluate apparent diffusion coefficients of polymers in mixtures with varying molar masses are pointed out.