Abstract The power conversion efficiency of the photoelectrochemical cell AuRhodamine BAu was found to be (2.4 ± 0.2) × 10 −6 per cent under typical operating conditions. A ten-fold improvement could be expected from constructional modifications, but further increases in efficiency would require changes in the cell chemistry. The low power conversion efficiency can be related to a low (4.7 × 10 −2 per cent) efficiency for the production of the open circuit photovoltage; a low (2.2 × 10 −2 per cent) quantum efficiency for the production of the short circuit current; and to markedly non-rectangular voltage-current characteristics. The deviations from a rectangular relationship and the low efficiency of current production arise from mass-transport limitations rather than from ohmic losses or activation polarisation. The low voltage efficiency probably arises from inefficiencies in the photochemical and electron transfer steps which lead to photovoltage production. The limitations of this type of cell as a solar energy conversion device are discussed.