The degradation of four phenolic acids by single UV radiation and by the advanced oxidation process constituted by the combination of ozone and UV radiation has been conducted. The phenolic acids, caffeic, p-coumaric, syringic and vanillic, were selected because they are major pollutants which are present in wastewaters from the production of olive oil and from wine distillery plants. In the single photochemical oxidation, the influence elf the operating variables is established and the quantum yields for each individual reaction are evaluated by means of a competitive kinetic method. In the combined ozone/UV radiation process, the generation of the hydroxyl radicals improves the degradation rate in comparison to the single oxidations performed : ozonation and UV photodegradation.