The present article describes preparation, characterization, and physical properties of nanoparticles-filled composites consisting of multiwall carbon nanotubes (MWCNT) and styrene-butadiene rubber and nitrile- butadiene rubber. The reinforcing MWCNT fillers were synthesized by chemical vapor deposition on iron and cobalt catalysts supported by calcium carbonate substrates. These MWCNT were further treated with nitric acid to produce hydroxyl and carbonyl functional groups on the carbon nanotubes (CNT) surfaces. Of particular importance is that these functionalized CNTs were found to exert profound influence on the elastomeric matrices, particularly the vulcanization activation energy, resistance to solvent swelling, enhanced glass transition temperature, and improved storage and loss moduli. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers