Abstract Ozone gas was dissolved in synthetic seawater at various concentrations. The toxicity of the resultant ozone oxidants on Brachionus sp. rotifers was noted. The time to 50% mortality ranged from 10 min for an oxidant concentration of 9.0 mg/L to more than 23 h for a 0.43-mg!L concentration. Sodium thiosulfate was added to water samples containing known toxic concentrations of ozone oxidants. One milligram of sodium thiosulfate neutralized 0.16–1.13 mg of ozone oxidants depending on the length of time the sample was aerated before thiosulfate was added: the longer the aeration, the less effectively thiosulfate neutralized oxidants. In samples in which the oxidants were fully neutralized by sodium thiosulfate, the time to 50% rotifer mortality was comparable to that in control samples never exposed to ozone.