Aims: To establish the influence of oxygen on Escherichia coli before, during and after exposure to UVA or simulated sunlight. Methods and Results: Bacterial suspensions were exposed either to UVA or simulated sunlight. Conventional aerobic plate counts of illuminated cell suspensions were consistently lower than those obtained under conditions where reactive oxygen species ( ROS) were neutralized, either ( i) by the addition of the peroxide scavenger sodium pyruvate ( 0.05% w/ v) to the medium with subsequent incubation in an anaerobic jar or ( ii) by culturing on a prereduced medium within an anaerobic cabinet, indicating that a substantial proportion of such cells are sublethally injured. While the presence of oxygen during the growth period resulted in a greater resistance of aerobically grown cells to simulated sunlight compared with their anaerobic counterparts, the extent of inactivation during illumination was directly related to the dissolved oxygen content of the water. Conclusions: The results show that, at each stage, oxygen has a marked influence on the observed colony count. Significance and Impact of the Study: Overall, the results indicate that future studies of bacteria exposed to UVA or sunlight should consider the effects of oxygen at every stage in the procedure, and especially during enumeration, where the inhibitory effects of ROS must be neutralized in order to obtain a valid count. An investigation of the effects of ROS neutralization on the counts of faecal bacteria under field conditions in natural waters is now required to establish the significance of these finding to solar water treatment.