Voltammetric techniques have been used to monitor the acid-base reactions of reduced forms of C-60 in dimethyl sulfoxide (DMSO) solution. In agreement with earlier work, the dihydrofullerene, C60H2, was found to show three reversible reduction processes with potentials about 0.1 V negative of the corresponding C-60 potentials in toluene/DMSO (67/33, v/v). For C60H2, the first and second acid dissociation constants are 4.7 and 16, respectively. Also, the reversible potentials for the C60H./C60H- and C60H-/C60H.- couples were found to be identical to the first two reduction potentials for C-60, within experimental error. The reversible potentials and pK(a) values were combined to compute the pK(a) of related species. For example, pK(a) for C60H. was computed to be 9. The titration of electrogenerated C-60(.-) with 4-nitrobenzoic acid (pK(a) = 9) was consistent with this calculation.