Electrochemical properties of eight methanofullerene films, prepared on electrodes by casting, were examined by means of cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) technique in acetonitrile solution. Compared with C-60 film, the film stability with respect to dissolution is associated with the length of the alkyl chains introduced into C-60 cage and is strongly dependent on the nature of the cation of the supporting electrolyte. TBA(+), Li+, K+, and Na+ were selected to observe the effect Of countercations on the electrochemical behavior of methanofullerene films. In TBAPF(6) solution, when short chains, such as formic ether or butyl butyrate groups, were introduced into C-60 cage, the film stability was less than that of C-60, When the length of the alkyl chain was extended to butyl 12 alkanoate group, the film would not dissolve until the third reduction process because the long alkyl chain could inflect and encase the cations into the film. In KPF6 or NaClO4 solution, however, the solubility of the salt formed by K+ or Na+ with methanofullerene anion was higher than that formed by TBA(+) with methanofullerene anion. Li+ (alkali metal) affected the behavior of the films in different ways. The possible electron-transfer mechanisms of methanofullerene films in various supporting electrolyte solutions were presented. Furthermore, the film images observed by atomic force microscopy indicated that TBA(+) and Li+ affected the behavior of the methanofullerene films in different ways. (c) 2006 Wiley Periodicals, Inc.