In the devolatilization step of polymn. processes, extruders are widely used for surface renewal. The LIST co-rotating processor (CRP) studied in the present paper, shows high performance for devolatilization processes. A sepn. model is correlated with exptl. data in the diffusion limited regime (where no foaming occurs). The effective viscosity range is 1000-6000 Pas, taking into account the influence of the shear rate. The model is applied with success to other types of kneading machines. [on SciFinder (R)]