Surface complexation modelling is introduced to investigate the surface reactions in a mixed galena and sphalerite flotation system. Computer-assisted calculations based on surface complexation modelling (SCM) are performed to obtain the mineral surface distribution of ethyl xanthate as well as carbonate and excess sulphide ions as a function of pH, The significance of pc is demonstrated by computing the mineral surface speciation in a flotation system with xanthate and excess of sulphide ions. The results are used to evaluate the influences of pH, pe, the concentration of xanthate, sulphide or carbonate ions to the mineral flotability. The flotation mechanisms in galena-sphalerite system an discussed based on SCM.