Two reasons for the poor performance of the iron–thionine photogalvanic cell are first the low solubility of thionine, which means that the incident solar radiation is absorbed too far from the illuminated electrode, and secondly the formation of dimers which do not undergo the desired photoredox reaction. The synthesis and photoelectro-chemical characterization of several isomeric disulphonated thionines are reported. All the isomers examined are sufficiently soluble to absorb solar radiation close to the illuminated electrode of a photogalvanic cell and show no evidence of dimerization via stacking interactions at concentrations up to 10^–3M. Using the following techniques, the transparent disc electrode, flash photoelectrolysis, and stopped flow, the kinetic parameters have been determined to see how well the conditions needed for an efficient photogalvanic system are met. The 4,6-isomer of disulphonated thionine is the only isomer that fulfills the required kinetic constraints.