The heterogeneous photocatalytic oxidation of fumaric, maleic and oxalic acids over TiO2 has been investigated. For aqueous suspensions at pH lower than the point of zero charge (pzc) Of TiO2, the photocatalytic degradation of the three studied diacids follows the Langmuir-Hinshelwood kinetic model, with the rate constant of the process decreasing in the order oxalic acid > maleic acid congruent to fumaric acid. At these low pH media, the adsorption of the organic diacids onto TiO2 particles is a key feature for their degradation, which is initiated by a photo-Kolbe process. For fumaric and maleic acids, a cis-trans isomerisation induced by the interaction between adsorbed molecule and semiconductor surface occurs. At pH's higher than the pzc of TiO2 the rate of oxalic acid oxidation decreases noticeably, while fumaric and maleic acids are both efficiently degraded in homogeneous phase by reacting with OH. radicals photochemically generated on the TiO2 surface, giving rise to a significant increment of both isomers degradation rate with increasing pH. At these pH's higher than the pzc of the TiO2, the three studied diacids show a very low degree of adsorption onto the semiconductor surface and no evidence of cis-trans isomerisation for both maleic and fumaric acids is detected. In accordance with the observed pH effects on degradation rate and over detected intermediates, a different mineralisation pathway is proposed as function of initial pH.