The formation and properties of redox active C-60/Pd films on gold electrodes of quartz oscillators were investigated by simultaneous cyclic voltammetry and piezoelectric microgravimetry at a quartz crystal microbalance. The films were prepared by electroreduction of solutions of C-60 and [Pd-II(CH3COO)(2)](3) in 0.1 mol dm(-3) tetra(n-butyl)ammonium perchlorate in acetonitrile-toluene (1:4, v:v). The composition of the solution from which the films were prepared significantly influenced the pattern of the film growth. The present results confirm that palladium clusters are codeposited with the C-60/Pd film ii the palladium complex to C-60 ratio was high. The reduced polymer film becomes partially electrochemically inactive at a sufficiently negative potential range. However, this electrically inactive film can be oxidized at very positive potentials. For charge compensation, the tetra(n-butyl)ammonium countereations enter the film during its electroreduction and are expelled from the film during electro-oxidation. At relatively high potential scan rates, only the outermost layers of the film that are in direct contact with the bathing solution are electrochemically active. At low scan rates, however, the bulk film material is also active. At very negative potentials, the film is removed from the electrode. The size of the tetra(n-alkyl)ammonium countereation is a major factor that determines both the electrochemical properties of the C-60/Pd films and their stability with respect to dissolution.