The dispersion of polyethylene and polypropylene in a polypropylene matrix using a co-rotating twin screw extruder has been investigated. Polymer pairs were selected to study the effect of viscosity ratio, defined as the viscosity of the minor component over that of the matrix, in the range of 0.1 to 900. The dispersion quality was defined by determining the number of "gels," i.e., large undispersed particles, present in thin films and by conventional microscopy techniques. The gel numbers were found to increase steadily with the viscosity ratio. It was also observed that particle size distributions in the high viscosity ratio blends was very broad, with particles as large as a hundred microns coexisting with much finer ones in the sub-micron range. For a given polymer blend, the re-processing was found to have an important effect on gel reduction. The effect of screw rotation speed, flow rate and minor phase feeding position was also investigated and is discussed in the paper.