An experimental investigation was conducted to evaluate the performance of three single-screw mixing elements: a Maddock; a Pineapple; and a Twente mixer. The study was designed to focus on the mixers＇ performance in the mixing of immiscible polymers in the molten state. Two independently controlled melt streams were fed to the mixer and the evaluation of the mixer performance was based on the following criteria: (i) the processing characteristics, namely the pressure drop/rise across the mixer for specific machine and material operating variables; and (ii) the resulting quality of mixing as determined by image analysis of phase morphology on samples collected at the die. The mixing performance was judged both qualitatively, in terms of spatial material distribution, as well as quantitatively, by analysis for the size of the dispersed phase. The blending tests were conducted using three pairs of LDPE/PS systems having different rheological properties. In addition to the experimental work, the flow in the Maddock mixer was simulated using a commercial finite element package (FIDAP). The calculations were done using the experimental conditions and the properties of a pure LDPE phase. The simulation results were used to explain the dispersion data obtained experimentally.