Malic acid, a model molecule for partially oxidized products found either in biomass or in various degradative processes, could totally be mineralized by photocatalytic degradation. Laboratory experiments enabled one to determine the identity of 15 intermediates, logically presented in four parallel degradation pathways. The major one is initiated by a decarboxylation (＇photo-Kolbe＇) reaction in the alpha-position with respect to the OH group. Parallel experiments have been performed in a slurry pilot solar photoreactor at PSA (Almeria, Spain), on a large scale (2501). The same main intermediate products and the same kinetic laws were observed. TiO2 has been used either in slurries or supported on several rigid inert plates. Deposition of titania on glass and quartz was carried out by a dip coating procedure and the deposition on stainless steel by an electrophoretic deposition process. The resulting materials have been exhaustively characterized by XPS, SEM/EDX, XRD and UV-vis absorption spectroscopy. Their photocatalytic activity pattern as a function of the nature of the support followed the decreasing order: TiO2/quartz > TiO2/steel = TiO2/glass much greater than photolysis. This decline in activity has been correlated with the presence of cationic impurities (Si4+, Na+ for glass and Cr3+, Fe3+ for stainless steel) in the titania layer as a consequence of the thermal treatment necessary to improve the cohesion of the titania layer and its adhesion onto the support.