Solar light photodegradation of three cinnamic acid derivatives (ferulic acid, caffeic acid and p-coumaric acid) present in olive oil industry wastewater has been achieved using 2,4,6-triphenylpyrylium hydrogen sulfate as an electron transfer photosensitiser. The parent cinnamic acid undergoes no photodegradation under the employed reaction conditions. The relative reactivities are in agreement with expectations based on qualitative reasoning: the substrates with two electron-donating groups, ferulic and caffeic acid, react faster than p-coumaric acid, which has only one activating substituent; while the non-activated cinnamic acid is nearly unreactive. The data obtained from fluorescence measurements show that the excited singlet state of the pyrylium salt is quenched by the acids, with the following rate constants: 3.6 x 10(9) M-1 s(-1) for cinnamic acid, 4.4 x 10(10) M-1 s(-1) in the cast: of p-coumaric acid, 8.7 x 10(10) M-1 s(-1) for caffeic acid, and 1.0 x 10(11) M-1 s(-1) for ferulic acid. These values are in good agreement with the order of reactivities observed in the preparative solar reactions,