Simultaneous photosensitized dechlorination and decarboxylation of isomeric mono- and dichlorobenzoic acids have been studied in the presence of naphthoxide ion in alkaline medium. The effects of the pH, concentration of the sensitizer, concentration of the substrate, the intensity of the light and the temperature on the rate of the dechlorination have been studied. The quantum efficiency of the photodechlorination has been evaluated. The mechanisms of the photodechlorination and decarboxylation have been suggested. The method is simple, economic and effective.