There are several reports indicating that deformation and breakup of dispersed drops is easier in extensional flow field than that in shear. This is particularly true for the systems containing the dispersed phase significantly more viscous than the matrix, e.g., blends in which the viscosity ratio λ – η disp /η matrix ≥3.8. These reports led to development of an extensional flow mixer, EFM, a device in which multicomponent, multiphase system [e.g., polymer alloys, blends, master-batches, filled systems] can be hydrodynamically mixed by flowing through a series of convergent/divergent regions of increasing intensity To be effective, EFM must be attached to a machine capable of melting and pressurizing the compound, preferably a single-screw extruder, SSE. In this paper efficiency of two compounding systems is compared, the first is made of a SSE and an EFM, while the second is a twin-screw extruder, TSE To evaluate the efficiency three types of blends, all characterized by high viscosity ratio, λ ≥ 3.8, were used: (i) high density polyethylene dispersed in polystyrene, HDPE/PS (ii) polypropylene impact-modified by addition of an ethylene-propylene elastomer, EPR/PP, and (iii) ultrahigh molecular weight polyethylene added to high density polyethylene, UHMWPE/HDPE System (i) was used to study the effect of compounding on blend's morphology – the dispersion from the SSE + EFM compounding unit was significantly finer than that from TSE. System (ii) was selected to examine usefulness of EFM for impact modification. The results demonstrated that SSE + EFM provided milder compounding conditions that less shear-degraded PP than TSE The impact strength of specimens prepared in SSE + EFM was superior to that of blends compounded in TSE. The system (iii) was studied to examine the relative merit of SSE + EFM over TSE to produce a finer dispersion of the UHMWPE domains that in turn would result in better dissolution of this ultrahigh molecular weight fraction. Again in this case the SSE + EFM compounder outperformed TSE – the dissolution of UHMWPE was significantly better without parallel degradation of the resin.