The supercritical carbon dioxide induced swelling and plasticization of polyamide 11 were investigated. The swelling kinetics exhibit an initial region of large swelling, in which the diffusion of CO2 into the polymer follows Fickian behavior, and a subsequent region of small volume increase that asymptotically approaches an equilibrium swelling value. The diffusion coefficient of CO2 in polyamide 11 was calculated from the initial slope of the swelling kinetics data. CO2, injected up to 3 wt % using an extrusion rheometer, is shown to be an effective plasticizer for polyamide 11, lowering the viscosity of the polymer melt by as much as 50%. The use of CO2 as a blowing agent was also investigated, and we report preliminary foaming attempts using a batch process. We obtained homogeneously distributed foams, featuring well-defined closed cells with an average diameter of 30 mum that had an unfoamed skin layer.