The reduction of p-benzoquinone (p-BzQ) has been studied at p- and n-InP electrodes. The p-InP/p-BzQ system exhibits a photocurrent density multiplication factor of 1.2 when the pH ratio of the systems is 4.0/7.0. This factor is constant at high and low light intensities. When the study is extended to higher pH values, e.g. using pH ratios of 8.0/8.3, 4.0/8.3 etc., photocurrent multiplication factors of 2 or more are observed. These results indicate that a possible pH-induced surface change is the factor responsible for photocurrent doubling and not the presence of the undissociated acid form of semibenzoquinone. The n-InP electrodes with p-BzQ were studied in detail at pH values of 7 and 4 with electrodes having carrier concentrations of 7 x 10(18), 4 x 10(18) and 5 x 10(15) cm-3. The ratios of the dark current density at pH 4 to that at pH 7 for the n-InP/p-BzQ systems ranged from 2 to 38 for the different n-InP electrodes. This wide range of dark current multiplication factors is possibly the result of a pH-induced surface change. The energy level diagrams of the interfaces suggest the participation of the conduction and valence bands in these processes.