A comparative study was carried out of the efficiency of titanium dioxide specimens Wackherr and Degussa P25 toward the photocatalytic degradation of picloram, a widely used herbicide. The study encompassed transformation kinetics and efficiency, identification of intermediates and reaction pathways. In the investigated range of initial concentrations (0.25-1.0 mM), the photocatalytic degradation of picloram in the first stage of the reaction followed approximately a pseudo-first order kinetics. The TiO2 Wackherr induced significantly faster picloram degradation than TiO2 Degussa P25 when high photocatalyst loadings and high substrate concentrations were used. By examining the effect of ethanol as a hydroxyl radical scavenger, it was shown that the photocatalytic degradation of picloram takes place mainly via the hydroxyl radicals, while the valence-band holes play a less important role. Total organic carbon analysis showed that complete mineralization of picloram occurred after about 16h of irradiation. Several degradation intermediates were identified by using LC-MS, GC-MS, and H-1 NMR, which allowed the proposal of a tentative pathway for the photocatalytic transformation of picloram. Finally, the photocatalytic degradation rates of three selected herbicides (picloram, triclopyr and mecoprop) were compared, and the results show that the efficiency of photocatalytic degradation is greatly influenced by the molecular structure. (C) 2011 Elsevier B.V. All rights reserved.