The photocatalytic degradation of diclofenac (DIC), 2-[2′, 6′-(dichlorophenyl)amino]phenylacetic acid is investigated under near UV–Vis and UV irradiation, using commercial TiO2 P25, synthesized TiO2 (anatase and rutile) and functionalized multi-walled carbon nanotube (MWCNTox): anatase (10-MWCNTox–TiO2) suspensions as catalysts. Factors affecting the kinetics of the process, such as the type and load of photocatalyst, and the presence of dissolved O2, or addition of H2O2 as co-oxidant have been compared. The degradation under UV irradiation is more effective than under near UV–Vis. Optimal conditions for a complete removal were obtained using synthesized anatase (0.5 g L−1) and 50% O2 (v/v) under UV irradiation, with rate constants ca. 0.9 min−1 (half-life time ca. 0.8 min). Eight photoproducts were observed from DIC photodegradation, mainly corresponding to photocyclisation (2-(8-chloro-9H-carbazol-1-yl)acetic acid, 1-chloro-8-methyl-9H-carbazole), decarboxylation (2,6-dichloro-N-o-tolylbenzenamine) and dehalogenation.Graphical abstractHighlights► A detailed mechanism is proposed for the photocatalytic degradation of diclofenac. ► The efficacy of nanostructured materials, including composites MWCNT–TiO2 is discussed. ► The mechanism of action of MWCNT–TiO2 composites is analyzed. ► The effects of the variables affecting the process are discussed.