Physisorption and desorption of organic compounds on a silicon surface in a two-component system consisting of isopropanol (IPA) and diethyl phthalate (DEP) were measured in situ by means of a quartz crystal microbalance and were analyzed based on the rate theory. The surface concentration in a steady state changed corresponding to the gas-phase concentrations of IPA and DEP. The measured surface concentration in a steady state was reproduced by calculation taking into account the larger limit of surface concentration which depended on the rate parameters of the organic compounds. The area occupied by the organic molecules, estimated in this study, indicated that the interaction between DEP molecules and IPA molecules on silicon surface was quite small in a two-component IPA-DEP system. (c) 2007 The Electrochemical Society.