BACKGROUND: The objective of the present study was to analyse the kinetics of photodegradation of three antibiotics from the tetracycline group (tetracycline (TC), chlortetracycline (CTC) and oxytetracycline (OTC)), and the influence of the operational variables: (1) initial concentration; (2) initial solution pH; (3) addition of hydrogen peroxide; (4) effect of the aqueous matrix (ultrapure water (UW), surface water (SW), groundwater (GW) and waste-water (WW) on these processes. RESULTS: The results obtained show that the photodegradation of the three tetracyclines fits first-order kinetics. The degradation rate depends on initial concentration and pH. Low concentrations of H2O2 markedly increased the efficacy of TC photolysis, with a linear relationship between degradation rate and H2O2 concentration for concentrations of 2 x 10(-2) to 2 x 10(-1) mmol L-1. The photodegradation rate is higher in real waters than in ultrapure water. The toxicity of oxidation by-products formed during tetracyclines photooxidation process was determined by a bioluminescent test, showing that toxicity increases during the process. CONCLUSIONS: Oxidation of tetracyclines by UV radiation alone is slow due to the low quantum yield determined. The UV/H2O2 process is an interesting alternative to oxidise tetracyclines in aqueous solution, because this process decreases total organic carbon concentration and tetracyclines oxidation by-products toxicity. (C) 2010 Society of Chemical Industry