Thermally stimulated current (TSC) measurements in the temperature range 90-400K were carried out on vapour-phase grown CdSxSe1-x single crystals over the full range of x (1.00, 0.90, 0.70, 0.50, 0.30, 0.10 and 0.00) by continuously exciting the crystals from different temperatures while cooling (TEWC) in the range 400-90K. Large changes in the peak currents were observed for different illumination conditions. The existing photochemical models are not able to explain the TSC dependence on TEWC observed in the crystals. A tentative photochemical model which takes into account the variation of photochemical reaction with composition has been proposed.