Adsorption isotherms of n-octane and toluene were determined by the gas chromatographic method on the surface of thermally treated and rehydroxylated silica gel. The same measurements were performed for a silica gel surface coated with boron oxide and borax. From the isotherms the maximal extrapolated film pressure (pi(max)) values for both adsorbates were determined, and the dispersion and polar components of surface free energy were calculated. The obtained values are discussed in terms of surface dehydroxylation and rehydroxylation. The derivative of the film pressure with respect to the adsorbed amount (deltapi/deltaa) was calculated for both adsorbates. The distribution of adsorption sites was also investigated and then correlated with surface free energy.