Perchlorate is a potent thyroid hormone-disrupting compound. Drinking water is one of the major sources of human exposure to perchlorate. Little is known about the occurrence of perchlorate in waters from China. In this study, water samples (n = 300) collected from 15 locations in 13 provinces and municipalities were analyzed for the presence of perchlorate. In addition, other inorganic anions that commonly occur in water--iodide, bromide, and nitrate--and the disinfection byproducts, bromate, chlorate, and chlorite were determined by high-performance liquid chromatography interfaced with tandem mass spectrometry. Perchlorate was detected in 86% of the samples analyzed, at concentrations ranging from <0.02 to 54.4 microg l(-1) (mean +/- SD 2.20 +/- 6.39 microg l(-1); median 0.62 microg l(-1)). Mean concentrations of perchlorate in tap water, groundwater, surface waters, and bottled water were 2.46, 3.04, 2.82, and 0.22 microg l(-1), respectively. Significant positive correlations were found between the concentrations of perchlorate and nitrate, perchlorate and chlorate, bromide and iodide, and nitrate and iodide.