Solution cast C-60 films were studied in aqueous solutions containing a variety of doping cations. Li+, Na+, K+, Rb+, Cs+, Ca2+, and Ba2+ were used as dopants and it was established that the reduction of the C-60 films shifted in parallel with the hydration energy of these cations. The reduction peak potentials also changed with the concentration of the cations, and this was assumed to be the consequence of the formation of reduced films which acted as cation exchange membranes. The cation exchange character was also detected after the reduction, but only with smaller ions (Li+, Na+), suggesting that, unlike the larger ions, they remain mobile in the films. The reduced C-60 films had quite different electrochemical activity and stability depending on the dopants. The most active and stable were those formed with K+ and Rb+ dopants. Some of the films (those doped with Li+, Na+ and Ba2+) could be oxidized electrochemically, while the others were probably oxidized only on their surfaces. In these cases an increased compactness and the formation of some protecting layers were assumed.