Adsorbed amounts from the mixture of nitrogen and oxygen on molecular sieve 5A were measured by static adsorption experiment. The experimental results were compared with the theoretical results calculated by ideal adsorbed solution theory(IAST) which treats the mixed adsorbate as an ideal solution in equilibrium with the gas phase and allows prediction of multicomponent equilibria from single-component isotherms alone. The equations resulting from IAST, however, must be solved iteratively even for the simple Langmuir isotherm. Three approximation methods were suggested in order to alleviate mathematical difficulties in the prediction of multicomponent equilibria, while making little or no loss in the accuracy of the calculation. Among the approximation methods, Padeapproximation and Taylor-series expansion methods gave excellent agreement with the exact results calculated by IAST.