While previous studies on a semiconductor-metal RRDE focused their attention mainly on steady-state conditions, in this study emphasis is on the transient currents at such a RRDE which are measured and analyzed under the effect of a light pulse. This behavior is studied on the basis of an existing theory for transient currents at an all-metal RRDE, using the analogy between a current step on a metal disc and a light step on a semiconducting disc electrode. Using this theoretical background, it is demonstrated that the analysis of transient currents on a semiconductor-metal RRDE is a useful new method for studying simple redox reactions and the adsorption and desorption of redox species at the semiconducting electrode. © 1981.1261-3241253