In this paper the electrochemical techniques (chronoamperometry, cyclic voltammetry and linear sweep voltammetry) have been employed in order to study the interactions between different mediators and the reaction center of the photosynthetic bacterium Rhodobacter sphaeroides. The experimental data are interpreted in view of the competitive occurring of an electrochemical reaction of the mediators at the electrode and their chemical reaction with the cofactors of the reaction center. The overall process is modelled by a set of differential equations, integrated using the Runge-Kutta method, that allow the calculation of k(ch) and k(el), the kinetic constant of the chemical and electrochemical reactions, respectively. The redox species used as mediators are the ferricyanide and 2,3,5,6-tetramethyl-p-phenylenediamine (DAD), both reductants of the photooxidized dimer, the Ubiquinone-0, which is able to reduce the quinones in the site A (Q(A)) and B (Q(B)) of the protein and the TMPD, an oxidant of the semiquinone anion. (C) 2000 Elsevier Science Ltd. All rights reserved.