The kinetic parameters of the cathodic reaction in a silicon-hydrofluoric acid system were measured using intensity modulated photocurrent spectroscopy (IMPS). The values and the potential dependences of the measured parameters were found to depend upon the concentration of dissolved oxygen. This is one of the reasons why photoelectrochemical measurements in this system have sometimes given poorly reproducible results. The effects of the presence of dissolved oxygen are discussed by considering the surface states and the relative energy levels of redox potentials in solution. (C) 2000 Elsevier Science Ltd. All rights reserved.