During studying the distribution of pollutants between the aqueous and particulate phases, an anomalous C s -effect phenomenon, namely, the experimentally measured partition coefficient (K p ) value of a given system decreased with increasing the sorbent concentration (C s ), was observed in a number of adsorption systems. Classical Langmuir model cannot predict the C s -effect. In this paper, a surface component activity (SCA) model was proposed, which suggests that surface component (adsorption site or adsorbed solute) activity coefficient is a function of C s because of the existence of adsorbent particle–particle interactions. A semi-empirical C s -dependent function of partition coefficient was derived, and it can predict the C s -dependence of partition coefficient for both organic compounds and heavy metals.