Photocatalytic degradation of atrazine was successfully and efficiently carried out in a large modular flow trough system under solar light. The degradation leads to complete mineralization of the alkyl substituents of s-triazine, giving trihydroxy-s-triazine as a final product. An efficient detoxification can be attained. The presence of 1 x 10(-3) M of peroxydisulphate, kept constant during the treatment, is sufficient to remove 98% of atrazine with very short residence times (less than 6 min), and to convert the parent triazine to more than 90% of trihydroxy-s-triazine in less than 2 h. The peroxydisulphate is particularly advantageous when large amounts of polluted ground water have to be treated. The factors affecting the total efficiency are discussed in view of a simple model for the primary events in the photocatalytic system.