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