The Langmuir equation, originally derived for gas-solid system, has been widely used to describe empirically solute adsorption from solution for almost a century. In this research, the Langmuir equation for solute adsorption from a liquid-solid adsorbed system was theoretically derived, taking into account various inteactions among components in liquid-solid adsorbed system, by using the stoichiometric displacement model for the adsorption of solute for the same system. The parameters in the Langmuir equation were related to that of the stoichiometric displacement equilibrium making it possible to describe solute adsorption under various concentrations of the solvent present. The equation was tested to be valid for the adsorption isotherm data obtained from different solvent concentrations. A good agreement between the values of the stoichiometric displacement parameter (Z) obtained from the isotherm data and those from high performance reversed-phase liquid chromatography was observed.