In this study, the heterogeneous photocatalytic degradation of prometryn using TiO 2 as photocatalyst was investigated. The main objectives of the study were: (I) to evaluate the kinetics of the pesticide disappearance, (II) to compare the photocatalytic efficiency of two different types of TiO 2, (III) to examine the influence of various parameters such as initial concentration of pesticide or catalyst and presence of oxidants (H 2O 2 and K 2S 2O 8), (IV) to evaluate the degree of mineralization and (V) to assess the detoxification efficiency of the studied processes. The experiments were carried out in a 500 ml pyrex UV reactor equipped with a 125 W high-pressure mercury lamp surrounded by a pyrex filter blocking wavelengths below 290 nm. Prometryn concentration was determined using HPLC. It was found that the degradation of the pesticide follows the first order kinetics according to the Langmuir–Hinshelwood model. Parameters like the type and concentration of the catalyst affect the degradation rate. A synergistic effect was observed when an oxidant was added in the TiO 2 suspensions increasing the reaction rate of photodegradation. In order to examine the extent of pesticide mineralization, DOC measurements were carried out. After 6 h of illumination, mineralization was achieved up to almost 70%. The toxicity of the treated solution was evaluated using the Microtox test based on the luminescent bacteria Vibrio fisheri, in order to compare the acute toxicity of prometryn and its photoproducts. The detoxification efficiency was found to be dependent on the studied system and it did not follow the rate of pesticide disappearance.