The evolution of blend morphology during compounding in a twin-screw extruder was investigated, putting emphasis on the effects of viscosity ratio, blend composition, and processing variables (barrel temperature profile and screw speed). For the study, we employed the following four blend systems: (i) polystyrene (PS)/poly(methyl methacrylate) (PMMA), (ii) PS/polycarbonate (PC), (iii) PS/high-density polyethylene (HDPE), and (iv) PS/polypropylene (PP). The choice of the above four blend systems was based on the difference in the melting temperature (T-m) of a crystalline polymer and the critical how temperature (T-cf) of an amorphous polymer. Here T-cf is defined to be approximately 55 degrees C above the glass transition temperature (T-g) and an amorphous polymer may be considered to flow at T greater than or equal to T-cf. The viscosities of the five polymers (PS, PMMA, PC, HDPE, and PP) chosen for melt blending were measured over a wide range of temperatures at shear rates ranging from 0.001 to 1000 s(-1). We conducted a ＇screw pullout＇ experiment to investigate the evolution of blend morphology, determined by transmission or scanning electron microscopy, along the extruder axis. We found that the initial blend morphology depends very much on the difference in T-cf or T-m between the constituent components and, also, on the viscosities of the constituent components. We observed that a co-continuous morphology was formed at the front end of the extruder, which then transformed into a dispersed morphology towards the end of the extruder. We found that the blend ratio determined the state of dispersion for asymmetric blend compositions and the viscosity ratio determined the state of dispersion for the symmetric blend composition.