Abstract Photolysis of benzenesulfonic acid(1) in an aqueous solution gave sulfurous acid, SO2, and/or sulfuric acid as inorganic products in a total yield of 38.6% based on the consumed 1 along with organic products. The organic products are mainly acidic tarry material containing sulfur(8.7%) together with a small amount of biphenyl (1%). Photolysis of an alkaline solution of 1 afforded a considerable amount of benzene (16%) and biphenyl(<1%). The photolysis is started by the C–S bond fission of 1 to give Ph· and ·SO3H radicals. The quantum yield for decomposition of 1 in acidic conditions is 0.26.