A new mixing device has been designed and developed in order to investigate complex mixing situations encountered in polymer blends and formulations. This mixing device called here rheo-mixer has been adapted on a classical rheometer and calibrated in terms of shear/stress that both permanent and dynamic regimes may be quantitatively run. The ability of the mixer to bring interesting information about complex polymeric systems was demonstrated by following the process of incorporation of a miscible liquid or partially miscible into a high viscosity polymer. As examples, EVA/diethyl 2-hexyl phtalate, poly(epsilon-caprolactone)/epsilon-caprolactone, and EVA/epsilon-caprolactone systems were tested. It was demonstrated that the viscosity ratio is not the predominant parameter for mixing process control because the mixing is mainly controlled by the diffusion for these low viscosity ratio systems (lambda < 10(-5)) as miscible systems (EVA/DOP and PCL/epsilon-CL) and partially miscible systems (EVA/epsilon-CL) are concerned. Actually, the mixing efficiency of these fluids is poor because the shear migration diminishes the rate of mixing due to a decrease of the deformation in the high viscous media. The mixing by thick striation is only effective when the viscosity ratio becomes, by the diffusion process, typically higher than 10(-3). Furthermore, the discussion based on the mutual diffusion coefficient agrees well with the experimental finding, at least qualitatively. (C) 2004 Elsevier Ltd. All rights reserved.