Smectic clays with quaternary ammonium salts (QAS) were subjected to hydrophobization, whereupon we applied the products in attempts to modify various polymers. The clays were of natural origin and contained a large proportion of montmorillonite and an original method of modification was applied. A significant improvement of the stability of unsaturated polyester (UP) resins, modified with clays modified with QAS, was observed with essentially no change in the reactivity of resins. The compositions of polyester resins containing 2 wt % of modified smectites had excellent thixotropy. The presence of modified bentonites in a cured commercial UP resin (2-4 wt %) improved its tensile strength (by 21-62%), Brinell hardness (23-70%), and unnotched impact strength (23-100%), depending on the type of bentonite used. The compositions of UP resins with modified clays had certain properties typical for nanocomposites: improved stiffness, strength, transparency, and fine-lamellar morphology of the fracture as found by electron scanning microscopy. (c) 2005 Wiley Periodicals, Inc.