The photodegradation in aqueous solutions of dimethyl 2,2 dichloro vinyl phosphate (common name DDVP or Dichlorvos), which is an organophosphorous insecticide, was studied by using illumination with UV lamp and solar exposure. TiO2 was used as a photocatalyst. In photocatalytic degradation, different TiO2 amounts, different illumination times and different pH values were examined in order to determine the optimal experimental conditions. DDVP removals at different initial DDVP concentrations under these conditions were investigated. In the photocatalytic degradation experiments, DDVP and PO43− (as orthophosphate) concentrations were determined. pH, potential and conductivity values were measured. © 1999 Taylor & Francis Group, LLC.