The effect of sulfite on the superadditivity of a phenidone-hydroquinone (P-Q) developer for the anodic formation of silver halide at pH 8.6 was studied using an anodically formed silver halide disk and a rotating ring-disk electrode. In the absence of sulfite the oxidation products of the developing agents were analyzed quantitatively at the ring electrode. But such products could not be detected at the ring electrode when sulfite was added to the developer, because sulfite reacted with quinone or semi-quinone to form hydroquinone monosulfonate which is not electrochemically active in the measured potential region. In such case, we found that the termination of the developing reaction could be detected by the potential change of the silver halide disk electrode. The period required for the termination of the development was dependent on the presence of sulfite. In case of P-Q developer the developing reaction was suppressed by the addition of sulfite. The electrochemical measurement based on the local cell mechanism for the development reactions indicates that the main effect of sulfite for the P-Q developing reaction is due to the suppression of the anodic part of the development.