The electrochemical kinetics for the oxidation of ferrocenemethanol (FeCH2OH) over the whole composition range of dimethyl sulfoxide (DMSO)-water solutions of different viscosities (eta) containing 50.0 mM (CH3)(4)NClO4 (TNIAP) at a Pt microelectrode was studied using scanning electrochemical microscopy (SECM). The measured diffusion coefficient. D, & lipH, as well as the standard rate constant of the heterogencous electron transfer, V, as a function of solution composition, showed a minimum at about a DMSO molar fraction (x(DMSO)) of 0.33, corresponding to the mixture with the maximum solution viscosity. The largest V value round, 2.06 +/-0.31 cm s(-1) in pure water (electrolyte) medium, was about 15 times larger than that obtained in the solution of x(DMSO) = 0.33 (0.14 +/- 0.02 cm s(-1)). A good linear correlation between In k(0) and In eta was observed within the solution composition range of 0.10 less than or equal to x(DMSO) less than or equal to 0.60. An excellent linear correlation between In k(0) and In tau(L), tile longitudinal relaxation time, was also obtained with a slope equal to 1.0 when x(DMSO) = 0-0.60. Unusually small rate constants found in tile solutions of x(DMSO) greater than or equal to 0.70 were attributed to adsorption effects at the tip and the substrate electrode. The k(0) obtained for the present system was generally found to be inversely proportional to the viscosity of the solution and directly proportional to tile diffusion coefficient of the electroactive species.