Results are presented for the dark and photoelectrochemistry of colloidal CdS particles studied with the optical rotating disc electrode (ORDE). The behaviour of the particles in the dark is governed by reversible electron transfer from a range of surface states superimposed over reversible electron transfer direct from the particle conduction band or shallow electron traps just below the conduction band edge. The oxidative current due to the latter effect exhibits a range of onset potentials arising from the polydisperse, Q-state nature of the particles under interrogation. The steady-state photoelectrochemistry varies from that in the dark in that (i) the surface states play a different role in determining the shape of the current voltage curve as they have been mostly oxidised by photogenerated valence band holes; and (ii) the photocurrent onset shifts to more anodic potentials as a result of a surface state-driven photocurrent attenuation and positive charge accumulation at the particle surface, both derived from hole-driven surface state oxidation. Light-on transient photocurrent measurements with the ORDE allow calculation of a pseudo-first-order rate coefficient for photogenerated electron loss, k(0), of similar to 0.07 s(-1). This suggests that the loss process being interrogated is not photogenerated electron-valence band hole recombination, but the loss of electrons left on the particle after some valence band hole filling reaction has occurred. Transient experiments also allow calculation of phi, the quantum efficiency for the photogeneration of electrons detectable by the ORDE. phi is found to be similar to 0.006, indicating that the dominant processes within the particle are direct and indirect photogenerated electron-valence band hole recombination.