The interface between CdSe and aqueous polysulphide solutions was investigated by impedance measurements. Variations in the flat-band potential as a function of solution parameters such as polysulphide concentration, hydroxide ion, added salt and cation effect were investigated. Furthermore, under illumination shifts in the flat-band potential were observed which were studied as a function of the above parameters. The findings of this investigation were as follows: (1) The flat-band potential was found to vary linearly with the redox potential of the polysulphide solution, which suggests a pinning of the Fermi level. (2) It was observed that the flat-band shifts were smaller in stabilizing solutions, i.e. charge accumulation at the interface led to appreciable shifts in the flat-band potential and to photocorrosion of the semiconductor. (3) Added hydroxide led to smaller flat-band shifts and to a larger stability of the photoelectrode. (4) Barium ion was found to have similar effects to that of caesium ion. (5) Adding salt to the polysulphide solution led to a decrease in the flat-band shifts of the photoelectrode. The results of this study are discussed in the light of recent models for the charge-transfer kinetics at this interface.