The effect of different chemical parameters on photocatalytic inactivation of E. coli K12 is discussed. Illumination was produced by a solar lamp and suspended TiO2 P-25 Degussa was used as catalyst. Modifications of initial pH between 4.0 and 9.0 do not affect the inactivation rate in the absence or presence of the catalyst. Addition of H2O2 affects positively the E. coli inactivation rate of both photolytic (only light) and photocatalytic (light Plus TiO2) disinfection processes. Addition of some inorganic ions (0.2 mmol/l) like HCO3-, HPO42-, Cl-. NO3- and SO42- to the suspension affects the sensitivity of bacteria to sunlight in the presence and in absence of TiO2. Addition of HCO3- and HPO42- resulted in a meaningful decrease in photocatalytic bactericidal effect while it was noted a weak influence of Cl-, SO42- and NO3-. The effect of counter ion (Na+ and K-) is not negligible and can modify the photocatalytic process as the anions. Bacteria inactivation was affected even at low concentrations (0.2 mmol/l) of SO42- and HCO3- but the same concentration does not affect the resorcinol photodegradation, suggesting that disinfection is more sensitive to the presence of natural anions than photocatalytic degradation of organic compounds. The presence of organic substances naturally present in water like dihydroxybenzenes isomers shows a negative effect on photocatalytic disinfection. The effect of a mixture of chemical substances on photocatalytic disinfection was also studied by adding to the bacterial suspension nutrient broth, phosphate buffer and tap water. (C) 2004 Elsevier B.V. All rights reserved.