The kinetics of the electro-reduction of the cercosporin phytotoxin in 1 M HClO4 + ACN is analyzed on the basis of the theory presented for the nine-member square scheme when protonations are assumed to be at equilibrium. Experimental results obtained fit fairly well the theoretical model proposed by Laviron for 2e(-), 2H(+) reactions. The formal heterogeneous rate constant, the voltammetric half wave potential and the cathodic transfer coefficient for the overall electrode process were determined from a fitting procedure of experimental square wave voltammograms by employing the COOL algorithm. Average values of 0.184 V, 0.45 and 0.019 cm s(-1) were calculated for the half wave potential, the cathodic transfer coefficient and the formal heterogeneous rate constant, respectively. The convolution analysis of cyclic voltammograms has been used to obtain the individual heterogeneous rate constants of the separate one-electron processes. Average values calculated were 0.013 and 0.008 cm s(-1), respectively. However, a complete description of the redox behaviour of cercosporin through the nine-member square scheme could not be achieved due to the lack of some thermodynamic parameters. An average value of 8 x 10(-6) cm(2) s(-1) was obtained for the diffusion coefficient of cercosporin from both chronocoulometry and convolution voltammetry measurements. Square wave voltammetry was also used to generate I-p versus c(cer)* calibration curves for this fungal metabolite. Detection limits of 5.8 x 10(-7) and 2.8 x 10(-7) M could be determined theoretically from calibration curves performed at 40 and 100 Hz, respectively, while minimal concentrations in the range of 1.9 to 3.8 x 10(-6) M could be measured experimentally by the same technique at those frequencies.