The mechanism of degradation of maleic acid in presence of Fe(Ill) in aqueous solution under UV-illumination has been investigated. The nature of detected intermediates depends on the type of the ferric salt being used: malic and malonic acids are detected when using iron sulphate, and acrylic acid when using iron chloride or iron perchlorate. Fumaric acid, generated by cis-trans isomerization process, as well as formic acid are detected in all cases. The degradation rate of maleic acid and its stable intermediates, that have been studied separately, follows a zero-order kinetics; the values of the rate constants depend on the nature of the carboxylic acid considered and on the type of Fe(III) salt used as source of Fe(111) in solution, indicating that the organic acid is reactive when it is co-ordinated with Fe(111). The role of oxygen has also been studied. From these experimental information, the mechanism of maleic acid degradation is discussed. (C) 2004 Published by Elsevier B.V.