Photosterilization of Escherichia coli cells was investigated using TiO2 particles in the presence of ferric ion as an electron acceptor. Deactivation of the cells was recognized in the reaction mixture containing ferric ion at the initial concentration of C-Fe3,C-0 = 1, 30 and 100 x 10(-3) mol/ml and the cell deactivation did not occur in the absence of ferric ion. The apparent sterilization rate constant, k', was determined from the photoreaction tests conducted under the varied conditions of average light intensity ((I) over bar (obs) = 0-22 W/m(2)) and TiO2 concentration (C-TO = 0-0.1 kg/m(3)). A proportional relation was obtained between the values of k' and (I) over bar (obs) with the slope of k'/(I) over bar (obs) = 3.8 x 10(-5) m(2)/J at C-TO = 0.01 kg/m(3), while the value of k'/(I) over bar (obs) increased with an increase in TiO2 concentration depicting a saturated profile up to C-TO = 0.1 kg/m(3) at (I) over bar (obs) = 8, 13 and 22 W/m(2). This relation between the values of k'/(I) over bar (obs) and C-TO could be kinetically explained by considering the photoreactions of ferric and ferrous ions and the adsorption of TiO2 particles onto E. coli cells in the reaction mixture.