Results are presented for the steady state and light-on transient photoelectrochemistry of two colloidal semi conductor/electron scavenger systems, US/Fe(CN)(6)(3-) and CdS/MV2+, studied with the optical rotating disc electrode (ORDE). Measurements with the ORDE allow calculation of values of 1.74 x 10(-9) and 3.12 x 10(-6) M s(-1) for the electrochemical rate coefficient for reduction of Fe(CN)(6)(3-) and MV2+, respectively, at the photoexcited particle surface. The difference in the values of k(ET) for the cationic and anionic scavenger species at the negatively charged CdS particles underlines the importance of the Coulombic interaction between particle and scavenger in determining the efficiency of particle-to-scavenger electron-transfer, ORDE measurements also indicate that the same Coulombic interactions are found to play an important role in determining the efficiency of the undesirable scavenger-to-particle back reaction, the reduced scavenger being oxidised by holes trapped at the particle surface. Importantly, the largest photocurrents are observed from the CdS/Fe(CN)(6)(3-) system which also has the lowest value for the rate coefficient for the scavenger-to-particle back reaction. The value of 0, the quantum efficiency for the photogeneration of electrons detectable by the ORDE, is found to increase upon the addition of either scavenger, However, the low values of phi observed (0.027 for the CdS/Fe(CN)(6)(3-) system, 0.02 for the CdS/MV2+ system) indicate that, even in the presence of an electron scavenger, the dominant processes within the particle are direct and indirect photogenerated electron-valence band hole recombination. (C) 2002 Elsevier Science B.V. All rights reserved.