A procedure for optimization of the cavity size for AM1-SCRF calculations of the electrode potentials in aqueous solution is presented, Comparison between the calculated electrode potentials of some benzoquinones and naphthoquinones obtained using the scaled cavity size and the experimental values suggests that the optimum cavity is placed at 1.4 times the van der Waals radii. Also, the electrode potentials of [2,5-bis(1-aziridinyl)-1,4-benzoquinones] with a wide range of substituents were calculated using the AM1-SCRF method, The calculated electrode potentials of these compounds using the optimum cavities give an r.m.s. error of 28 mV compared with an r.m.s. of 49 mV for the values obtained using the recommended radii for SCRF calculation.