Pilot plant experiments applying solar titanium dioxide photocatalysis and solar photo-Fenton treatment at different pH and iron concentrations with an initial diclofenac concentration of 50 mg L-1 are described. In preliminary experiments absence of hydrolysis and slow photolysis under solar irradiation of diclofenac solutions were observed. Solar photo-Fenton treatment with freshly precipitated iron at pH around 7 showed first order kinetics, the reaction taking place on the surface of the iron precipitate. Simultaneous oxidation, precipitation and re-dissolution processes of diclofenac governed photo-Fenton decomposition kinetics at pH 2.8. The use of different iron concentrations (0.03-0.75 mM) showed no influence on the reaction rate in a neutral medium due to reactor geometry. Similar behaviour (no influence of iron concentration) was observed at pH 2.8, due to precipitation problems. A pH of around 4. close to the pK(a), of diclofenac, showed promising results, partly overcoming both iron and diclofenac precipitation. Solar titanium dioxide photocatalysis with Degussa P-25 followed first order kinetics and no precipitation or adsorption occurred. Decomposition of diclofenac took around 100 min under all photo-Fenton treatment conditions employed. Decomposition by titanium dioxide photocatalysis took about 200 min. In photo-Fenton treatment, hydrogen peroxide consumption to diclofenac decomposition was about 80-110 mM at pH 2.8 and 40 mM in the other two treatments (neutral pH and pH = 4). © 2005 Elsevier B.V. All rights reserved.