We report a nanoparticulate composite TiO₂–BaTiO₃ film which exhibits an increased antibacterial photocatalytic activity under visible light. The pure BaTiO₃, TiO₂ or their mixture do not attain a significant photocatalytic capacity under visible light. However, when these oxides are simultaneously synthesized under controlled conditions the resulting crystals exhibit a high catalytic effect. The effect of this material on microorganism lysis is studied and the mechanisms for the observed damage are investigated. Attenuated total reflection Fourier transform infrared spectroscopy provides the evidence of chemical changes (formation of carbonyl and carboxylic groups) in the cell membranes under visible light by TiO₂–BaTiO₃ but not by TiO₂. Cyclic voltammetry demonstrates that peroxidation occurs in the absence of UV light and in the presence of TiO₂–BaTiO₃ but not in the presence of TiO₂ alone. Atomic force microscopy reveals the morphological changes of the cells in this process.