The photocatalytic inactivation of Escherischia coli and Pichia Pastoris was studied with combustion synthesized titanium dioxide photocatalysts. Three different combustion synthesized (CS) catalysts were used viz., CS-TiO2, 1% Ag substituted in TiO2 and 1% Ag impregnated in TiO2. All the combustion synthesized catalysts showed higher activity as compared to the activity observed with commercial Degussa P-25 TiO2. The effect of various parameters like catalyst loading, different catalysts and initial cell concentration was studied. At the optimum loading, 1% Ag impregnated TiO2 showed the maximum efficiency and complete inactivation of both the microorganisms was observed within an hour of irradiation. The morphology of inactivated cells was studied by inverted microscope and SEM. From the images obtained, it was hypothesized that damage to the cell wall was the main cause of cell inactivation. The initial cell concentration had a prominent effect on the inactivation. At a low initial cell concentration, the complete inactivation of E. coli and P. pastoris was observed within 10 and 20min, respectively. This shows that P. pastoris has a stronger resistance towards photocatalytic inactivation than E. coli. The inactivation reactions were modeled with power law kinetics. The order of reaction in case of E. coli and P. pastoris were determined as 1.20 and 1.08, respectively. © 2010 Elsevier B.V.