Abstract: The influence of S2－ ion and its concentration upon the photoinduced electron transfer and redox reaction of substrates on the surface of modified CdS colloid have been studied by means of ESR technique.The experimental results show that the existence of S2－ ion with high concentration is advantageous to the interfacial photoreduction reaction and may restrain the photooxidation reaction.If the redox potential E0 of the substrate is negative,photoreduction reaction is impossible to occur in the system with low concentration of S2－ ion,but it can take place as the concentration of S2－ ion is sufficient high; when E0 of the substrate is positive,photooxidation reaction can be prohibited entirely under condition of high S2－ ion concentration,even though it is able to occur while the concentration of S2－ ion is low.Due to the strong interaction of S2－ ion with the surface of CdS superfine particles,negative shift in flat potential Efb of CdS leads to a change of gap position,and then to a variation in match relation between the redox potential E0 of the substrate and the position of conducton band to valence band in semiconductor particles.According to Langmuir's adsorption isotherm model,a formula expressing the relation between the concentration c of modification compound and the negative shift ΔEfb in flat potential was derived:ΔEfb=Δ Kc/(1＋Kc).By using this formula,experiment facts may be explained rationally,the higher the concentration,the greater the amount of negative shift ΔEfb,and thus the more favourable to photoreduction reaction.Therefore,by choosing suitable modification substance and adjusting its concentration,the selection,direction as well as the mechanism of interfacial photoreaction can be controlled effectively.