The efficiency and cost-effectiveness of H2O2/UV for the complete decolorization and mineralization of wastewater containing high concentrations of the textile dye Reactive Black 5 was examined. Oxidation until decolorization removed 200-300 mg g(-1) of the dissolved organic carbon (DOC). The specific energy consumption was dependent on the initial dye concentration: the higher concentration required a lower specific energy input on a weight basis (160 W h g(-1) RB5 for 2.1 g L-1 versus 354 W h g(-1) RBS for 0.5 g L-1). Biodegradable compounds were formed, so that DOC removal could be increased by 30% in a following biological stage. However, in order to attain 800 mg g(-1) overall mineralization, 500 mg g(-1) of the DOC had to be oxidized in the H2O2/UV stage. A cost analysis showed that although the capital costs are much less for a H2O2/UV stage compared to ozonation, the operating costs are almost double those of ozonation. Thus, while H2O2/UV can compete with ozonation when the treatment goal only requires decolorization, ozonation is more cost-effective in this case when mineralization is desired. (c) 2006 Society of Chemical Industry.