The adsorption equilibria of n-heptane, n-octane and n-nonane on silicalite and ZSM-5 have beenmeasured in the temperature range of 373.15--473.15K under low pressure (0－5.332kPa). All the experimentaldata can be represented by a generalized characteristic curve of the extended adsorption potential theory utilizingthe parameter of the initial heat of adsorption, which is estimated reasonably by a new approach.