Aims: To improve solar water disinfection using a photocatalysing semi-conductor and to study the mechanisms involved in this process. Methods and Results: Cells of Escherichia coli were used as the microbiological indicator to study the possibility of improving the efficiency of solar water disinfection using titanium dioxide (TiO2 ) as a photooxidizing semi-conductor. TiO2 was used either as a suspended powder or in an immobilized form. Both applications improved the efficiency of solar disinfection. TiO2 in suspension was more effective than the immobilized form, producing enhancement factors of 1.62 and 1.34, respectively. The concentration of TiO2 greatly affected efficiency, with a maximum effect at 1 mg ml(-1) . Higher TiO2 concentrations reduced the efficiency. Dimethyl sulphoxide (DMSO) and cysteamine (Cys), hydroxyl radical (OH. ) scavengers, were used to elucidate the mechanisms involved in the presence of TiO2 . Both DMSO and Cys totally abolished the enhancing effect produced by the presence of TiO2 . Conclusions: Sunlight has a potential water disinfecting capacity. The use of TiO2 greatly improved this efficiency. The effect of TiO2 was mainly concentration-dependent, giving maximum efficiency at 1 mg ml(-1) . The presence of DMSO and Cys removed the TiO2 -induced enhancement, indicating that OH. may be involved in the process of cell killing. Significance and Impact of the Study: The efficiency of solar disinfection is limited and time-consuming and needs to be improved. The use of a semi-conductor is promising as it reduces the time of exposure and therefore increases the efficiency of solar disinfection. This would allow for the availability of good quality water, and hence would improve the quality of life.