The photocatalytic inactivation of Gram negative Escherichia coli and Gram positive Lactobacillus helveticus by both TiO2 and ZnO with 365-nm ultraviolet (UV) light was studied in a batch reactor. Almost all the initial E coli cell (10(8) CFU/ml) were inactivated in 40 min in the presence of 2 g/l ZnO. Photocatalytic inactivation of bacteria was found to follow first order kinetics with the highest rate constants being 4.5 x 10(-1) and 2.2 x 10(-1)/min for E coli and L. Helveticus, respectively, treated with 2 g/l ZnO. A similar bactericidal tendency was found in the UV-TiO2 system, where the rate constants were 3.7 x 10(-1) and 1.8 x 10(-1)/min for E coli and L. Helveticus, respectively. The use of air as the purging gas, providing a stronger oxidising environment to both strains, gives a higher catalytic effect on bacteria inactivation than that of nitrogen. (C) 2003 Elsevier Science Ltd. All rights reserved.