Adsorption isotherms of 21 kinds of gases and vapors were measured on an activated carbon or a silica gel, and affinity coefficients were determined by applying the DR equation to the isotherms measured. Structure-activity parameters consisting of atomic and bonding parameters are proposed to correlate the affinity coefficients on activated carbon and silica gel. The adsorptive index was assumed to be given by the arithmetical summation of atomic and bonding parameters by taking into account the structure of the adsorbate. The values of atomic refractions reported in the literature are used as the atomic parameters, and the bonding parameters were determined by the Marquardt method so that the adsorptive index correlated experimental affinity coefficients in the present and literature results. It is found that the affinity coefficient is equal to the ratio of adsorptive index of adsorbate to that of a reference adsorbate.