BACKGROUND: The effect of four dye-auxiliary chemicals, typically employed in acid dyeing, on the performance of UV/H2O2 decolouration of the model non-biodegradable dye C.I. Acid Orange 7 (AO7) was investigated. The initial concentration of AO7 was 0.150 gL(-1), while the concentration of the auxiliary compounds (NaCl, Na2SO4, Na2CO3 and CH3COOH) was varied in the range 1-10 g L-1. RESULTS: The negative influence of the presence of the dye-auxiliary compounds studied on the decolouration rate of AO7 decreased in the following order: CH3COOH > Na2SO4 > NaCl > Na2CO3. Results were quantified in terms of the observed kinetic rate constant, k(obs) (s(-1)), of AO7 decolouration as a function of dye auxiliary chemical concentration. The decolouration rate of AO7 decreased as the concentration of dye-auxiliary compound increased in the range 1-5 g L-1, while higher concentrations had a minor effect. Upon addition of 5 g L-1 of CH3COOH, NaCl and Na2SO4, the kinetic rate constant decreased by 39%, 30% and 12%, respectively. CONCLUSIONS: It was concluded that the presence of NaCl, Na2SO4 and above all of CH3COOH should be considered in the design of the treatment of real dye-bath effluents by UV/H2O2 (c) 2008 Society of Chemical Industry.