The photocatalytic degradation of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and simazine (2-chloro-4,6-diethylamino-1,3,5-triazine) herbicides widely used in agriculture has been investigated. Experimental design methodology was used to assess the influence of pH and TiO(2) concentration and the efficiency of the process. The results indicated that in the experimental domain investigated TiO(2) concentration was the most significant factor in contrast to the scarce influence shown by the initial pH. The mechanism of atrazine and simazine photocatalytic degradation was studied under the experimental conditions determined as optimal. No full mineralization of atrazine or simazine was achieved. The main intermediates occurring during the reaction were identified by high performance liquid chromatography (HPLC). Based on the concentration profile of intermediates formed during the treatment a degradation pathway is proposed for each herbicide. The toxicity along the reaction was monitored by means of luminescence bioassays using Vibrio fischeri. The inhibition percentage decreases with the irradiation times what can be associated with the formation of highly hydroxylated intermediates. (C) 2010 Elsevier B.V. All rights reserved.