Titanium dioxide photocatalysed water disinfection is induced by the interaction of light with TiO2, which generates highly reactive free hydroxyl radicals (OH). These free radicals create lethal damage that leads to bacterial death. Normally, decreasing TiO2 particle size increases the area of light interaction. This may possibly increase the concentration of OH generated and hence increases disinfection efficiency. Moreover, decreasing the particle size increases the force of attraction between the particles and cells, which could create aggregates that may contribute to the local OH concentration. In the present investigation cells of Escherichia coli were used as the test microorganism, TiO2 as the photocatalyst and sunlight as the light source. Four different surface areas of TiO2 particles corresponding to 10, 50, 80–100 and ≥300 m2 g−1 were tested at a concentration of 1 g l−1. Disinfection efficiency increased with increasing the surface area producing a maximum between 80–100 m2 g−1 followed by a reduction at ≥300 m2 g−1. The reduction in the efficiency at this relatively high surface area was attributed to the increase in the local concentration of OH. This increase may be high enough to initiate radical-radical interaction that would compete with bacterial cells and reduce the chance of bacterial cell-radical interaction taking place. Moreover, the phenomenon of TiO2 aggregation with bacterial cells plays an important role, and the extent of aggregation increases with decreasing particle size. Such aggregation could augment the concentration of OH within the cell vicinity. This suggests that surface area is a key factor in determining the efficiency of disinfection, and that concentration is a vital factor.