The photodegradation of 1,5-dihydroxynaphthalene catalyzed by meso-tetra(4-sulfonatophenyl) porphyrin (H2TPPS4-) under visible light irradiation in aerated aqueous solution has been studied and the main product has been identified as 5-hydroxy-1,4-naphthoquinone. It is found that the photodegradation is dependent on the pH of reaction system and the addition of sodium azide (NaN3) leads to a significant decrease in the degree of 1,5-dihydroxynaphthalene photodegradation. The species distribution of H2TPPS4- in solution was obtained from potentiometric titration experiments. It is suggested that the mono-protonated monomeric species [H(H2TPPS)](3-) is the main active species of H2TPPS4- to catalyze 1,5-dihydroxynaphthalene photodegradation. (C) 2008 Elsevier B.V. All rights reserved.