As a first step toward the development of photocathodes for use in water-purification reactors, we have investigated the behaviour of p-type Si(1 0 0)-oriented single crystals and glow-discharge a-Si:H layers as photocathodes for the reduction of dissolved dioxygen. The electrode surfaces were prepared by HF rinsing and were characterised by cyclic voltammetry in the dark and under tungsten-halogen lamp illumination. This study has been performed in dilute-buffered fluoride medium, in dilute sulphuric acid, and lastly in three buffered phosphate media at pH 2, 7 and 12. A study using a rotating-disk electrode (RDE) arrangement permits to distinguish the contribution from dioxygen reduction (mass-transport dependent) and that from hydrogen evolution (kinetically limited). Crystalline Si and a-Si:H appear to exhibit rather similar behaviours. The Si surface acquires catalytic properties for dioxygen reduction upon negative pre-polarisation. However, these catalytic properties do not appear usable in practice for efficient operation, which calls for chemical modifications of the silicon surface. (C) 2004 Elsevier Ltd. All rights reserved.