Solar light photodegradation, catalysed by a pyrylium salt, of seven benzoic acids present in olive oil mill, has been studied. Significant percentages of photodegradation (20-40%) have been achieved after 6 h of solar exposure for six of the acids, even though they were expected to be difficult to oxidise, due to the presence of an electron-withdrawing carboxylic acid group directly attached to the aromatic ring. Quenching constants for the electron-transfer process between the substrate and the excited catalyst were calculated by means of fluorescence measurements for syringic acid (66 x 10(9) M-1 s(-1)), gallic acid (51 x 10(9) M-1 s(-1)), veratric acid (51 x 10(9) M-1 s(-1)), vanillic acid (48 x 10(9) M-1 s(-1)), protocatechuic acid (37 x 10(9) M-1 s(-1)) and p-hydroxybenzoic acid ( 15 x 10(9) M-1 s(-1)); no quenching was found for benzoic acid. These photophysical measurements are in good correlation with the yields obtained in the pyrylium salt photocatalysed degradation of those phenolic acids. (C) 2001 Elsevier Science B.V. All rights reserved.