Abstract Differential and integral entropies were derived as functions of amount sorbed for CH 4, C 2H 6, C 3H 8, n-C 4H 10, iso-C 4H 10 and neo-C 5H 12 in a mixed sodium-potassium form of the synthetic zeolite L. The analysis showed that these sorbates possessed less thermal entropy when sorbed than in their bulk liquids. It was not possible to describe the sorption isotherms over the complete range in degree of filling, θ, of the intracrystalline pores in terms of any single model isotherm. However, equations for localised sorption were often valid over considerable ranges in θ. The isotherm equation based on an osmotic virial equation of state for the sorbate in the mixed sorbent-sorbate phase provided a much more successful description. Equilibrium and other constants were derived for several vapours according to this isotherm.