In the present work, rubber/clay nanocomposites were prepared by a solution mixing process using fluoroelastomers and different nanoclays (namely, Cloisite NA+, Cloisite 10A, Cloisite 20A, and Cloisite 30B). Fluoroelastomers having different microstructure and viscosity (Viton B-50, Viton B-600, Viton A-200, and VTR-8550) were used. Characterization of the nanocomposites was done by using X-ray diffraction and atomic force microscopy. The mechanical and dynamic mechanical properties were studied. The surface energy of the clays and the elastomer was also measured. Even with the addition of only 4 phr of clay in Viton B-50, tensile strength and modulus improved by 30-96% and 80-134%, respectively, depending on the nature of the nanoclays. Exfoliation was observed with both the unmodified and the modified clays at low loading in all the fluoroelastomers. Best properties were observed with the unmodified clay. All the grades of fluororubber followed the same trend. The increment (19%) in storage modulus was also higher in the case of the unmodified clay filled Viton B-50 system. The results were explained with the help of thermodynamics, surface energies, and swelling studies. The difference in surface energy, Delta gamma, between the rubber and the unmodified clay was lower. The work of adhesion (67.63 mJ/m(2)) between Viton B-50 and Cloisite NA+ was also higher than that (51.42 mJ/m(2)) between Viton B-50 and Cloisite 20A. Negative Delta H-S value for the unmodified clay-filled system thermodynamically favored the formation of the nanocomposite as compared to the modified clay filled samples where Delta H-S is positive or zero. (c) 2005 Wiley Periodicals, Inc.