This study evaluates the degradation of a selected p-lactam antibiotic (oxacillin) by TiO2 photocatalysis, sonochemistry, the photo-Fenton process and electrochemistry (with a Ti/IrO2 anode in sodium chloride). The four oxidation processes were able to successfully degrade the pollutant. Although in the three advanced oxidation processes the hydroxyl radical was involved in the degradation mechanism, significant differences with respect to the degradation pathways, by-products generated and degree of mineralization were observed. In the electrochemical treatment, the main oxidation route was active chlorine attack, whereas for the sonochemical and photo-Fenton systems the hydroxyl radical played the determinant role. In TiO2 photocatalysis, the combined action of direct oxidation by the holes and the attack of hydroxyl radicals removed the antibiotic. As a consequence, analysis of the by-products showed that all of the four oxidation processes exhibited thioether moiety oxidation, secondary amide breakdown and opening of the beta-lactam ring. However, decarboxylation of the pollutant was only found with TiO2 photocatalysis, which can be explained by direct oxacillin oxidation by the holes. The oxacillin isomerization pathway was promoted using the electrochemical process. While the photo-Fenton and TiO2 photocatalysis treatments also showed hydroxylation at the aromatic ring, due to the different degradation routes, the characteristics of the systems and the nature of the by-products generated, a different extent of mineralization was found. The removal of total organic carbon in TiO2 photocatalysis and the photo-Fenton system was similar to 90%. and similar to 35% respectively, but with the sonochemical and electrochemical treatments the pollutant was not mineralized. (C) 2015 Elsevier B.V. All rights reserved.