Photochemical degradation of benzotriazole is investigated in aqueous solution at 25 degrees C and pH values varying in the range 3-11. The quantum yields at different pH values are evaluated by analysing the kinetics of the reacting system. Reaction kinetics are markedly affected by pH because of its influence upon benzotriazole dissociation. A mathematical method that takes into account the presence of intermediates and of the pH has been developed.