In the photoreaction of Escherichia coli IM303 (superoxide dismutase (SOD)-deficient mutant) and MM294 (wild-type strain) with TiO2 particles, the viability of strain IM303 decreased linearly with photoreaction time, while the time profile of the viability of strain MM294 exhibited a curved form. Using strain MM294 with varied initial SOD activities, the TiO 2 photoreaction tests were conducted at incident light intensities of I-0=4,8 and 14W m(-2), and the time profiles of bacterial viabilities were analyzed on the basis of the series-event model. The value of n (corresponding to the step number in the series reaction kinetics described by the model) increased with an increase in initial SOD activity (A(SOD,0)), giving a mean value of A(SOD,0)/n=7.1 x 10(-9)U cell(-1) under the conditions examined. SOD activities in the cells of strain MM294 with A(SOD,0) = 1.9 x 10(-8) and 4.0 x 10(-8) U cell(-1) decreased with the progress of photoreaction conducted at I-0 = 14 W m(-2). The transition of intracellular SOD activities expressed was in agreement with the observed data by considering the changes in bacterial cell populations with varied SOD activities based on the proposed model.