The phenomenon of ozone-induced particle destabilization was studied employing a colloidal suspension of 150 mg/L sodium montmorillonite (Na-M) suspended in river water containing 3.1 mg/L natural organic matter(NOM). The suspension was treated with high (approximately 105 muM) and low (approximately 10 muM) ozone doses. Extended DLVO theory was utilized to investigate the surface thermodynamics of unozonated and ozonated Na-M coated with NOM (cNa-M). Ozonation decreased the surface charge and the Lewis base parameter, and increased the Lewis acid parameter and Lifshitz-van der Waals component of the surface energy. The overall result of these changes was a decrease in the change in the total free energy of interaction and hence a decrease in the stability of cNa-M with increasing ozonation. Modification in the surface thermodynamics responsible for destabilization could possibly be attributed to partial dealuminization of Na-M and associated NOM transformations, increasing the autophilicity of the colloids.