The partial photocatalytic degradation of maleic acid has been investigated with the purpose of elucidating the mechanism of catalyst action for some of the early transformations. In particular, it is proposed that the photocatalytically induced cis-trans isomerization of maleic acid and fumaric acid is initiated by adsorption-dependent reductive electron transfer. An investigation into the involvement of superoxide in the oxygenation reactions observed near neutral and higher pH clearly demonstrates that superoxide does not initiate the chemistry. (C) 2004 Elsevier B.V. All rights reserved.