Heterogeneous kinetics for the electrooxidation of N,N,N＇,N＇-tetramethyl p-phenylendiamine (TMPD) to its corresponding radical monocation (TMPD.+) on Pt disk ultramicroelectrodes was studied in binary mixtures of acetonitrile (ACN) with dimethylsulfoxide (DMSO), dimethylformamide (DMF), methanol (MeOH), ethanol (EtOH) and propan-2-ol (Pr-2-OH). Values of the formal rate constant (k(f)(o)) at different mixture compositions were calculated through cyclic voltammetry at high-scan rates by applying the well known Nicholson method. It was found that changes in the formal rate constant with solvent composition may be explained through differences in the solvation of the activated complex and can be reasonably described by a two-step solvent exchange mechanism. On the other hand, experimental results were also analyzed by performing multiparametric as well as simple linear regressions between In k(f)(o) and differnet solvent-dependent parameters, which affect both the exponential and the pre-exponential terms in the general expression for k(f)(o). The dependence of k(f)(o) on the solvent longitudinal nlaxation time was verified through the viscosity values measured at different mixture compositions by considering that values of the longitudinal relaxation times for these mixtures are not available in the literature. Moreover, the dependence of k(f)(o) on the Pekar factor and solvation parameters was also verified. O 2000 Elsevier Science Ltd. All rights reserved.