The degradation of phenacetin (PNT) by the combination of low-pressure mercury lamp and chlorine (UV/chlorine), an advanced oxidation process (AOP) of recent interest, was systematically investigated in terms of degradation kinetics, effects of chlorine dosage and water parameters, oxidation products as well as toxicity evaluation. The degradation of PNT followed pseudo first-order kinetics. The first-order rate constant (k(obs)) in the UV/chlorine AOP was 4.3, 8.4, and 11.1 times that of dark chlorination, UV/H2O2, and UV/PS, respectively, with the same molar dosage of oxidant at pH 7.2. Radical quenching tests suggested that chlorination, % OH and reactive chlorine species were responsible for the UV/chlorine oxidation of PNT with contributions of 26.33%, 14.6% and 59.07%, at pH 7.2. As chlorine dosage gradually increased from 100 to 500 mu M, the corresponding k(obs) monotonically increased from 0.0229 to 0.216 min(-1). k(obs) was not apparently affected by the pH and coexisting chloride, but decreased by 56.5% and 75.4% in the presence of 10 mM HCO3- and 10 mg/L NOM. Slight decreases (around 15%) of k(obs) occurred in the raw water and tap water tests, while little effect was observed in filtered water samples compared with ultrapure water tests. Six typical disinfection byproducts including trichloromethane (TCM), chloral hydrate, dichloropropanone, trichloropropanone, trichloronitromethane, and dichloroacetonitrile were detected. The TCM yields increased to 159.95 mu g/L within 20 min reaction of UV/chlorine, comprising 13.3% of the degraded PNT (per mole of carbon). The acute toxicity to luminescent bacterium Q67 by UV/chlorine was lower than chlorination under similar reaction conditions.