The effects of chemical treatments on red mud (RM) were investigated in terms of thermal stabilities of PMMA/RM and PVC/RM nanocomposites. N2/77 K adsorption behavior and contact angles were studied in the pore structures and surface energetics of RM, respectively. Thermal stabilities of the nanocomposites were investigated using a thermal mechanical analyzer (TMA) and thermogravimetric analysis (TGA). As a result, the acidically treated RM (ARM) had higher adsorption properties, including specific surface area, than untreated RM (VRM) or basically treated RM (BRM). A change in the structure of the ARM surface was due to hydrolysis or leaching a metal salt out of RM. Also, the electron acceptor (gamma(S)+, acid) of ARM and the electron donor (gamma(S)-, base) of BRM were increased in the development of acid and basic functional groups, respectively. PMMA/ARM nanocomposites had higher thermal stability and mechanical interfacial properties than PMMA/VRM or BRM nanocomposites. These results were due to the improvements of the dispersion properties and acid-base interfacial interactions of basic PMMA and ARM. In this work, although the dispersion properties of the BRM decreased, the thermal stabilities and mechanical interfacial properties of PVC/BRM nanocomposites increased, which could be attributed to improvement in the interfacial interactions between acidic PVC and BRM.