A mathematical model for a quasi-reversible EC mechanism at the surface of spherical macro- and micro-electrodes is presented for the case of square-wave voltammetry. The analysis considers the influence of the chemical and the electrochemical kinetics as well as the contribution of sphericity to the voltammetric responses. The apparent electrode sphericity and the global apparent reversibility are jointly affected by the variation of frequency (f). If the electrode radius (r(o)) is smaller than 30 mum, but the sphericity contribution is neglected, it is not possible to obtain good kinetic information. For E-sw > 0.2 V the system is shifted far from the steady-state and the k(chem) value can be analysed regardless of the k(s) value. However, for E-sw = 50 mV, it is not possible to evaluate k(chem) if k(s) < 10(-3) or K > 1. Simple equations for the dependence of DeltaPsi(p) on f, r(o) and E-sw have been provided. Nevertheless, the analysis of such complex electrochemical systems should require not only a study of these dependences, but also fitting of the theoretical voltammograms with experimental results to provide some useful mechanistic information.