Mixed doping of C-60 fullerene films on gold substrates was carried out by electrochemical reduction in aqueous solutions, producing a variety of electrodes. It was established that the ratio of the built-in dopants (cations) can be controlled on the solution side by adjusting the ratio of the concentrations of the different cations. Ion-exchange was proposed as the determining step. From the experiments of mixed doping, conclusions on the existence of transient species, and on the mechanism of the reduction of fullerene films, were also drawn.