The adsorption isotherms for SF6 and C3H8 are reported at three temperatures near ambient on pillared clays with different gallery sizes imposed by heat treatment. X-ray diffraction experiments show a decrease in pore size (interlayer spacing) with increasing calcination temperature. Heat treatment also causes the decrease in surface areas and micropore volumes obtained from N-2 adsorption isotherms. The relationship between the two effects is discussed. An objective, mathematically rigorous, method to calculate isosteric enthalpies of adsorption is proposed. This method allows for a critical assessment in the error of the derived enthalpies based on estimates of the error in the laboratory measurement. Isotherms with complex shapes can be successfully analyzed over the entire range of the experimental variable. Using this approach, a decrease in initial enthalpies of adsorption with heat treatment is observed.