The evolution of blend morphology during compounding in an internal mixer was investigated using transmission electron microscopy and scanning electron microscopy. Emphasis was placed on investigating the effects of viscosity ratio, blend composition, and processing variables (temperature, rotor speed, and mixing time) on the evolution of blend morphology in five blend systems: (i) nylon 6/high-density polyethylene (HDPE), (ii) poly(methyl methacrylate) (PMMA)/polystyrene (PS), (iii) polycarbonate (PC)/PS, (iv) PS/HDPE, and (v) PS/polypropylene (PP). These blend systems were chosen on the basis of the difference in the melting temperature (T-m) between two crystalline polymers (nylon 6/HDPE pair), the difference in the 'critical how temperature' (T-cf) between two amorphous polymers (PMMA/PS and PC/PS pairs), or the difference between the T-cf of an amorphous polymer and the T-m of a crystalline polymer (PS/HDPE and PS/PP pairs). The T-cf of an amorphous polymer is de facto equivalent to the T-m of a crystalline polymer in that from a rheological point of view an amorphous polymer may be regarded as being a 'rubber-like solid' at temperatures below T-cf and a 'liquid' at temperatures above T-cf, which is approximately 55 degrees C above the glass transition temperature (T-g) of an amorphous polymer. We observed a co-continuous morphology in PMMA/PS, PC/PS, PS/HDPE and PS/PP blends when the melt blend temperature was above the T-g, but below the T-cf of the constituent amorphous components, and a dispersed morphology when the melt blending temperature was increased far above the T-cf of the constituent amorphous components. Further, we found that the formation of a co-continuous morphology depends on blend composition and the viscosity ratio of the constituent components at a specified melt blending temperature. Most importantly, we have reached the conclusion that a co-continuous morphology is a transitory morphological structure that appears when a phase inversion takes place from one mode of dispersed morphology to another mode of dispersed morphology. The mode of a dispersed morphology is found to depend upon the blend composition and the viscosity ratio of the constituent components.