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Inverse gas chromatography was used to study the adsorption of water and ethanol on to a starchy (wheat flour) and a cellulosic (wheat straw) substrate, in the temperature range 50–90 °C, before and after their regeneration by thermal treatments at 140 and 170 °C, for 12 and 24 h each. From the chromatographic retention data it was possible to calculate the separation factor (s) of the two solutes and to obtain values for Gibb's free energy (ΔGs) and enthalpy (ΔHs) of adsorption. The results showed that water was adsorbed more strongly than ethanol by both substrates, at all temperatures, before and after their regeneration. In addition, it was found that, for both untreated and thermally treated materials, the adsorption of both solutes was stronger at lower temperatures. Thermal treatment had no effect on the adsorption of ethanol on either substrate. In contrast, the adsorption of water was weaker for both substrates regenerated at 140 °C and even weaker for the substrates treated at 170 °C, irrespective of the duration of the regeneration process. Furthermore, at both regeneration temperatures, the decrease in water sorption was approximately the same for both materials, irrespective of regeneration time. The separation factors followed, in general, the same trend observed for water sorption. They had lower values for materials regenerated at 170 °C for 24 h. Both materials were equally affected by the regeneration process.