We applied the rate theory of plastic deformation to evaluate the flow activation volume in polystyrene melts and in its composites with different concentrations of multiwall carbon nanotubes. The experiments consisted in the analysis of a perturbation induced to the melt by varying the shear rate of a monotonic test at a melt state with nearly constant viscosity. The variation consisted on the application of shear steps with lower shear rate during short-time intervals. The material's response to the perturbation allows a flow activation volume to be evaluated. For the pure polymer, the flow activation volume compares to the volume of a tube confining the chains, confirming the validity of tube model. Addition of nanotubes to the melt affects the melt response in the high temperature flow region, resulting in the development of a plateau in G'(omega). This plateau is linked to interactions of chains with nanotubes. However, the flow activation volume was not affected by the addition of nanotubes, suggesting that tube model holds also for composites of polymer melts with carbon nanotubes. (C) 2011 Elsevier Ltd. All rights reserved.