Homogeneous photo-oxidation of anthraquinone-2-sulfonic acid sodium salt (ASS) with H2O2 and Fe3+ was investigated. The continuous photoproduction of Fenton reagent can be achieved in this way. The behaviour of Fe3+ was similar to Fe2+ under the same experimental conditions and the photoactivity was favourably influenced in the presence of oxygen. The photo-Fenton reactions greatly accelerated ASS degradation in comparison with direct photolysis using Pyrex flasks (lambda > 290 nm). The influence of Fe3+, Cu2+, H2O2 concentration, pH, temperature and gas atmosphere was systematically studied. A solution containing ASS 3.10(-3) M was degraded up to ca. 90% under light (AM 1) at 60 degrees C in 3 h and in ca. 5 h at 35 degrees C. Complete oxidation was achieved after 15 h. About 90% of the total organic carbon was degraded to carbon dioxide when dearomatization was completed. Photo-Fenton systems could be used for initial treatment of waste waters containing ASS to obtain aliphatic and oxidized compounds susceptible to being more easily degraded in biological waste water treatment plants. Reaction products were characterized by total organic carbon (TOC); nuclear magnetic resonance (NMR); gas chromatography (GC) and spectroscopy (OD).