An analysis of decomposition of methylene blue (MB) in an aerated aqueous solution by its action spectrum has shown that sulfur-doped titania (S-TiO2) has activity under visible-light irradiation and that the decomposition mechanism depends on the excitation wavelength. This suggests that photosensitive materials are not suitable as probe chemicals for photocatalytic activity tests, especially those for evaluation of activity under visible light. It is confirmed using acetic acid instead of MB that S-TiO2, but not ordinary non-doped titania, leads to the photocatalytic reaction under visible radiation below 600 nm, which corresponds to its absorption spectrum. (c) 2006 Elsevier B.V. All rights reserved.