p-Coumaric acid has been used as a probe in order to study the effect of solar light catalysed by 2,4,6-triphenylpyrylium salts on phenolic compounds present in olive oil industry wastewaters. The results are very satisfactory, and important degradation yields are achieved. Methylene blue has also been used as a photocatalyst, but it results in slower degradation. Other advanced oxidation methods (ozone and/or UV radiation) have been tested as well; as expected, p-coumaric acid abatement is much faster (100 times), but ozone and UV are dangerous and expensive for industrial uses. In contrast with other phenolic acids, ozone and UV do not show an important synergistic effect in p-coumaric acid oxidation. This could be due to differences in the absorption spectra. Major p-coumaric acid oxidation intermediates have been identified and quantitated by HPLC; on the basis of these data, a reaction mechanism is proposed.