An extended form of Gibbs’ definition of the dew point is experimentally demonstrated to be valid. The adsorption K-values of methane-n-butane mixtures on a porous, hydrocarbon-like polymer substrate are shown to coincide With the known vapor-liquid K-values at five different dew point conditions. It is thereby shown that a chromatographic column packed with that substrate provides a new method to determine the dew point of a hydrocarbon gas mixture with greater precision than conventional methods. Five adsorption isotherms are presented. From the total number of moles adsorbed at the dew point, the adsorption layer thickness is shown to be less than 5 A or 1 layers of butane molecules at the higher pressures.