Equilibrium adsorption isotherms were obtained for methane, ethane and propane in nitrogen on a 5A molecular sieve and also for methane and carbon dioxide on 5A and 4A molecular sieves at 25° C. The single component experimental data of these gases agreed well with the empirical Langmuir and Freundlich models. A statistical thermodynamic model also represented the data fairly well. Ternary isotherms for methane, ethane and propane mixtures and binary isotherms for methane and carbon dioxide mixtures were obtained experimentally. A modified extended Langmuir model correlated the adsorption of ternary and binary mixtures fairly well. The Freundlich-type multi- component model gave a reasonable fit for ternary mixtures and represented the adsorption of binary mixtures fairly well. Other models were also considered for both ternary and binary mixtures. Breakthrough curves for single and multicomponent mixtures were obtained and mathematical model s investigated to establish which was the most suitable model. A finite difference technique was used to solve the mathematical models. Surface diffusion resistance was found to play a dominant role and could be the rate controlling step for single component adsorption. An equilibrium control model does not predict the breakthrough curve for a ternary mixture of methane, ethane and propane very well, but did give a reasonable fit for binary mixtures of methane and carbon dioxide. Experimental adsorption-desorption cycles showed that this technique could be used to separate ethane as well as propane from a ternary mixture of methane-ethane and propane, and to separate carbon dioxide from a binary mixture of methane and carbon dioxide. Adsorption-desorption cycles showed that separation was achieved and that it depends strongly upon the instant at which desorption is commenced. The equilibrium model was used to describe both adsorption-desorption cycles for binary and ternary mixtures.