A new method for developing adsorption equilibrium relations for gas mixtures is presented. First, pure component isotherms are measured and described by appropriate isotherm equations. Then, binary measurements are performed with one component at a trace level and the other at a substantial loading, a condition we refer to as Henrys law behavior with a second component in excess. The virial excess mixture coefficient method is used to obtain correction terms to the ideal adsorbed solution theory, with the coefficients determined solely from the Henrys law data. Then, for data measured over the full composition space, the theory provides simple and accurate improvements to predictions of the ideal adsorbed solution theory. The method is implemented for the system of nitrogen and oxygen adsorbed on LiLSX zeolite.