Chlorate and chlorite concentrations were determined in water samples taken from 33 swimming pools. In the pools under investigation, disinfection of the water is carried out either by gaseous chlorine (n = 14) or hypochlorite solution in conjunction with flocculation and sand filtration. A number of the pools also use ozone treatment to augment the disinfection process. Chlorite was not detectable in any of the samples (detection limit 1 mg/l). High concentrations of chlorate were detected in samples from a number of the pools; in one case as high as 40 mg/l. Higher chlorate concentrations were found to be associated with those pools using hypochlorite solution as a disinfecting agent. In contrast, relatively low chlorate concentrations were found in pools treated with gaseous chlorine. In order to elucidate any relationship between the chlorate content of pool water and that of the respective hypochlorite stock solution, chlorate and bromate concentrations were determined in the hypochlorite stock solutions of nine pools. Bromate concentration in the stock solutions were not found to exceed 1.2 g/l, chlorate was measured in concentrations of up to 44.5 g/l. The additional use of ozone as part of the water purification process appears to have no significant influence on chlorate concentration. Chlorate has no bactericidal properties and does not interfere with the measurement of certain parameters relevant to hygiene in swimming pools such as free and combined chlorine, pH or redox potential. At present, the effects of high chlorate concentrations in swimming pool water are unclear. Our initial investigations indicate that chlorate has no cytotoxic (Neutral-Red assay) or irritating properties (HET-CAM assay). However, both chlorate and chlorite are known to interfere with the haematopoetic system. In Germany, the MCL for chlorite in drinking water is 0.2 mg/l. It is therefore strongly recommended that measures should be taken to reduce chlorate concentrations in swimming pool water.