A promising system for the photodissociation of water into H2and O2involves the zinc porphyrin photosensitised reduction of methyl viologen and, for practical purposes, it is essential that the yield of primary redox products is maximal. The yield of redox products depends upon the partitioning of an ion-pair, and the efficiency of charge separation increases with increased electrostatic repulsion between the ion products. Where there is strong electrostatic repulsion within the ion-pair, the ionic strength of the aqueous solution influences the rate of ionic separation and high yields of redox products are favoured at low ionic strengths. These experimental observations are explained in terms of a kinetic model.