This study is to predict adsorption equilibrium data for binary gas mixture systems using the vacancy solution model(VSM). The single component adsorption isotherms of propane and n-butane on a 5A molecular sieves(Davison C-625) were determined experimentally and from these isotherms the parameters in a single component adsorption isotherm equation by the vacancy solution model(n₁^s∞, b₁, Λ₁₃, Λ₃₁)were ca;ci;ated. A nonlinear regression method and an op-timization method were employed in the calculation of the parameters. The parameters were then used in the predic-tion of the binary gas mixture adsorption equilibrium curve through a trial-and-error method. The experimental mixed-gas adsorption data for varying composition of propane-n-butane mixture were in good agreement with the predicted equilibrium curve based on the VSM.