Inactivation of B. subtilis spores with ozone was investigated to assess the effect of pH and temperature, to compare the kinetics to those for the inactivation of C. parvum oocysts, to investigate bromate formation under 2-log inactivation conditions, and to assess the need for bromate control strategies. The rate of B. subtilis inactivation with ozone was independent of pn. decreased with temperature (activation energy of 42,100 J mol(-1)), and was consistent with the CT concept. B, subtilis was found to be a good indicator for C. poruum at 20 30 degreesC, but at lower temperatures R. subtilis was inactivated more readily than C. parvum. Bromate formation increased as both pH and temperature increased. For water with an initial bromide concentration of 33 mu gl(-1), achieving 2-logs of inactivation, without exceeding the 10 mu gl(-1) bromate standard, was most difficult at 30 degreesC for B. subtilis and at midrange temperatures(10-20 degreesC) for C. parvum pH depression and ammonia addition were found to reduce bromate formation without affecting B. subtilis inactivation, and may be necessary for waters containing more than 50 mu gl(-1) bromide. (C) 2001 Elsevier Science Ltd. All rights reserved.