Based on the vacancy solution theory, VST, and using the NRTL activity coefficient model, a new adsorption isotherm is developed. Following VST, the vapor-solid equilibrium system is considered as a solution consisting of the adsorbate and adsorbent where the adsorbate is the solute and vacancies play the role of the solvent. The NRTL equation is used to accounting the non-ideality or the solute-solvent interaction in the vacancy solution. The parameters of the proposed model are calculated by using the optimization of experimental data for the different single gas adsorption systems at various temperatures. The new isotherm is used to correlate the isotherms of 12 systems, each one at different temperatures. The accuracy of the present isotherm is presented by the average absolute deviation percent (AAD%) and compared with those results obtained by using Wilson equation. It is shown that the proposed model correlates the equilibrium pressure with an average absolute deviation of 3.77%, which is less than the deviation obtained from the original VST which is 10.16%. (C) 2010 Elsevier B.V. All rights reserved.