Abstract: The surface free energy components of clay-organic complexes were determined to assess to what extent an organic adsorbate modified the surface properties of the mineral, insofar as the stability of soil aggregates is concerned. Adsorption isotherms for two synthetic, humic acid-like polymers were determined on a Ca-montmorillonite. From contact-angle measurements performed on dry surfaces, the surface free energy properties of the clay-organic complexes were determined using the two-liquid-phases method (water and hydrocarbons). This method allows both the dispersive and nondispersive components of the solid surface free energy, γ S D and γ S P , to be determined. The results show that a very small amount of polymer (1% by weight) adsorbed on the external surfaces of the montmorillonite decreased markedly the surface free energy components of the clay: γ S D decreased from 75 to 28 mJ/m2 for polycondensate catechol (PC) and from 75 to 30 mJ/m2 for polycondensate catechol triglycine (PCT), whereas γ S P ranged from 35 to 16 mJ/m2 (PC) and from 35 to 17 mJ/m2 (PCT). Although their chemical compositions were different, both polymers similarly modified γ S D and γ S P . Increasing the amount of polymer adsorbed (from 1% to 3.5% by weight) affected mostly γ S P , which became as low as 5 mJ/m2; meanwhile, γ S D decreased from 30 to 23 mJ/m2. Possibly, the molecular orientation of the adsorbate changed in the process of dehydration. Following adsorption of synthetic humic acid-like polymers, dry Ca-montmorillonite complexes displayed γS values <50 mJ/m2, which were consistent with the solid-water contact angles measured in air.