The formation of a polyethylene/polyurethane blend polymer via in situ polymerization of diols and diisocyanate monomers dispersed in a molten polyethylene matrix was investigated in an internal mixer. Two urethane systems were used in this study. The evolution of the blend morphology with increasing molecular weight of the polyurethane phase was discussed. The difference in viscosity between the minor and major phases has a controlling influence on the qualitative compounding behaviour exhibited. Furthermore, the viscosity ratio appears to be the physical parameter which governs the blend morphology development. The phase inversion was observed at ηdghm = 0.05. Beyond the phase inversion point, Wu's model (Polym. Eng. Sci., 1987, 27, 335) quantitatively predicts the decrease of the size morphology with increasing molecular weight of the polyurethane phase. Nevertheless, high solubility of isocyanate monomers and low solubility of alcohols in molten polyethylene induce an imbalance stoichiometry which limits the degree of polymerization (molecular weight) during the blending process. A pre-polymerizing step of the urethane system limits this solubility phenomenon.