The photoelectrochemical characterisation of highly photosensitive and semi-insulating sintered pellets of CdS doped with Cu and Cl has been carried out. In the course of this work, we have observed a complete suppressin of the dark current but a pronounced photocurrent under cathodic polarisation. Both these observations indicate an unusual cathodic (i.e. forward bias) behaviour. It has been shown that due to a decrease in series resistance under illumination by an extrinsic photoconductivity effect, the “ideal” cathodic behaviour, i.e. normally found in dark for an n-CdS electrode with low resistivity, can get restored. We have also observed a red shift in the anodic photoelectrochemical spectral response of these samples. It is in agreement with our previous photoelectrochemical studies on CdS: Cu: Cl electrodes and can be explained by considering a Cu impurity excitation process. Additionally, we have observed that the background illuminations as such (employed in the “double-beam” technique of spectral response determination) do not alter the overall nature of the spectral response but result in an interesting photocurrent enhancement phenomenon. These photocurrent enhancement observations have been explained by taking into account the photoeffects at Cl-compensated Cu acceptor centers possessing a high capture cross section preferably for photoexcited holes.