Inverse gas chromatography has been used to evaluate the adsorption parameters (Delta H-a, Delta H-st, Delta S-a and Delta G(a)) of some probe molecules, each representing a class of organic (n-hexane, cyclohexane, benzene, n-octane, 1-octene and isooctane) on bentonite and chemically treated-bentonites. The adsorption parameters of the probes on the bentonite samples were determined in infinite dilution region. Adsorption of the organic species was investigated in the temperature range of 200-275 degrees C, using a flame ionization detector, and nitrogen as a carrier gas. The net retention volumes (V-n) of the probes were determined by the help of the retention times (t(R)) observed on gas chromatograms for each probe. Injection was made at least three times for each probe, obtaining reproducible results of +/- 0.5%. It was found that benzene exhibits more negative Delta H than for n-hexane and cyclohexane on all of the adsorbents. In addition, it was found that 1-octene exhibits more negative Delta H than for n-octane and isooctane on the chemically treated-bentonites, whereas n-octane exhibits more negative Delta H than for 1-octene and isooctane on the natural bentonite. Also, interactions of benzene with the natural- and chemically treated-bentonites were found to be stronger than those of n-hexane and cyclohexane with the same carbon number. Again, interactions of the 1-octene with the chemically treated-bentonites were found to be stronger those of n-octane and isooctane with the same carbon number. On the contrary, interactions of n-octane with the untreated-bentonite were found to be stronger than those of 1-octene and isooctane.