The entanglement effects on the phase separation of polystyrene/poly(vinyl methyl ether) (PS/PVME) blends were studied by considering the viscoelastic relaxation time and the early stages of dynamic demixing. Particular interest was focused on the blend in which the molecular weight of one component is much higher than that of the other component (M-w, (PS) >> M-w, (PVME)) The viscoelastic relaxation time of high molecular weight PS and PVME in the blends can be clearly separated and the onset of the phase separation process after a temperature jump is delayed to a later time. We suggest that this is due to the formation of a temporary network of the well entangled polymer which needs to relax before phase separation can occur. The results are in good agreement with the theoretical work reported previously by Clarke et al. [Macromolecules 1997;30:4459]. For the blends containing low molecular weight linear and 3-armed star molecules, the viscoelastic effects on the phase separation are not significant since the polymers are not effectively entangled.