The efficiency of the iron thionine photogalvanic cell for solar energy conversion is severely limited by the insolubility of thionine. Six new modified thiazine dyes have been synthesised and investigated to see if they meet the photochemical and kinetic requirements for the photogalvanic cell. A linear free energy relation between the rate constant for the thermal back reaction and the standard electrode potential of the thiazine dye has been found. The implications of this relationship together with the variation of pH, of [FeIII] and of the solubility on the efficiency are explored. For efficient energy conversion it is also necessary to have a selective electrode; results are presented for the thioninecoated electrode which show that this modified electrode is suitable for the cell.