A detailed study of barrier modulation mechanism of photoconductive response has been carried out in modified Schottky barrier contacts of gold to conductive CdS. The forward bias I-V characteristics are shown to be well described by the usual thermionic diode law with a light dependent barrier height V//B plus V//H. The barrier height and its change with illumination minus 0. 23 V in a typical diode at high light intensity, have been determined by photoemission and capacitance measurements, as well as from the voltage and temperature dependences of ln I. The limiting performance of this type of photodetector is discussed.