Composites based on natural rubber (NR) and multiwall carbon nanotubes (MWNTs) were prepared by using different mixing processes. The state of dispersion was evaluated by transmission electron microscopy and the extent of reinforcement imparted to the elastomeric matrix was assessed by equilibrium stress-strain measurements. Dynamic mechanical analysis was performed in order to get information on the polymer-filler interface. Molecular investigations including infrared analysis under strain or Raman spectroscopy were carried out in order to point out possible alterations in the crystalline phase or interfacial interactions between the filler and the matrix.