This paper outlines the basic events that occur when a semiconductor, in contact with aerated liquid water containing low concentrations of pollutants, is photoexcited. First, the factors favouring this recombination of photoproduced charges are recalled, and the difficulties in decreasing the recombination are underlined. Second, the formation and identification of the species resulting from charge transfer with O2, H2O and pollutants are presented. Third, the reactions involving these species are considered, and methods for assessing their respective importance are critically reviewed. Fourth, on the basis of the view that the photocatalyst surface is covered by tightly bound water layers which hinder the adsorption of many organic pollutants, the hypothesis according to which primary photocatalytic events can take place within the near-surface solution layers is discussed.