The effects of the incorporation of singlewalled carbon nanotubes (SWNTs) on the physical and mechanical properties of natural rubber (NR) are described. Characterization of these new materials has been performed by dynamic mechanical analysis, differential scanning calorimetry, and Raman spectroscopy to obtain information about of the possible interactions between both materials as well as the dispersion of SWNTs on elastomer matrix. The results are then compared with those obtained for NR-carbon black composites. Dynamic mechanical analysis indicates a stronger filler-matrix interaction in the case of SWNTs incorporation, showing a noticeable decrease of the height of tan 5 peak, as well as a marked shift of T towards higher temperatures. In particular, the increase of the storage modulus indicates a beneficial effect of SWNTs incorporation with respect to NR filled with carbon black and the pristine polymer matrix. In addition, calorimetric analysis indicates that both fillers accelerate the NR vulcanization reaction, this effect being more evident when SWNTs are added into the matrix. Raman spectroscopy indicates that SWNTs dispersion into the elastomer matrix creates residual strain on the nanotubes bundle. We demonstrate that the Raman microprobe technique provides a means for load transfer effectiveness of SWNTs. (C) 2004 Wiley Periodicals, Inc.