The dispersions of both reactive and nonreactive polymer-polymer blend systems achieved in three different mixers are compared. The dispersions are pre pared using an industrial scale twin-screw extruder, a laboratory internal mixer, and a miniature cup and rotor mixer. The morphology development in the three mixers is remarkably similar : The dispersed phase is stretched into sheets and ribbons; these sheets and ribbons then break into cylinders, which subsequently break into droplets via Rayleigh-type instabilities. Drop size distributions can be accurately predicted if we know the size of cylinders formed in the high shear fields of the mixer. There is a significant effect of quenching time on blend morphology-i.e, to properly evaluate mixing, blends must be quenched extremely quickly (well within a minute). Otherwise, we need to consider the morphology development during the quenching time, which may not be relevant to the mixing. There is a uniform shear stress in the miniature mixer, unlike the other mixers, which have varying stress levels. It is shown that a high stress level followed by a lower stress level is required in polymer blending to achieve efficient mixing. In the high stress level, the dispersed phase is stretched into extended shapes, which undergo instabilities and break up upon entering the low stress level. In the miniature mixer, the dispersed phase sees only one stress level, and thus very extended shapes persist at the end of mixing. The final dispersions in the twin-screw extruder and internal mixer at matched maximum shear rates are almost identical. For similar shear rates in the miniature mixer, the final dispersion of reactive blends is comparable to the other mixers. However, the miniature mixer does a poor job in dispersing high viscosity uncompatibilized blends, and the mixing conditions must be altered to obtain efficient mixing.