Thermally induced phase separation in PS/PVME blends was studied by solid state NMR. The proton spin—lattice relaxation in both the laboratory and the rotating frame were measured for the entire range of blend composition. Under conditions in which the blends are said to be compatible by other techniques T results obtained at −5°C showed microheterogeneity at a 10−°A scale. T1pH values at room temperature are closer to the longer relaxation time of PS than that expected from simple weighted average of the relaxation times of the constituent homopolymers. This indicates incomplete averaging by spin diffusion and a restraining effect of PS on segmental motions of PVME. The blends were heated to cause phase separation and quenched. From the biphasic decay of 13C magnetization, the compositions of the separated phases were estimated to give a lower critical solution temperature phase diagram. NMR relaxation in PVME blends with PS molecular weights of 9, 100, and 900 K were compared.