Based on the principle of solution thermodynamics for liquid-solid equilibrium, a simple mathematical expression has been developed to express liquid adsorption isotherms. A weight fraction based activity coefficient model has been derived for the solid phase nonideality. The proposed model has been evaluated and compared with four other models commonly used for liquid adsorption isotherms in the literature. Systems used in this study are solute adsorption from dilute aqueous solution on various activated carbon or molecular sieve. For 14 systems at various isotherms for the temperature range 0.1-75 degrees C and the solute concentration range of 5-26540 g/m(3) for 382 data points, the proposed model predicts equilibrium concentration with an average deviation of 6%. The proposed model clearly outperforms other available methods such as vacancy solution theory, exponential model and various other modified forms of the Freundlich isotherm. The unique features of the proposed model are its simplicity, generality and accuracy over the entrie experimental concentration and temperature ranges.