A solid technique to synthesize genuine diblock copolymers of poly(vinyl methyl ether) (PVME) and polystyrene (PS) for the phase behavior study was developed by the coupling reaction between end-chlorinated PVME and living PS anions. The phase behavior of the block copolymer with deuterium-labeled PS (PVME-b-DPS) was investigated by the small-angle neutron scattering (SANS) technique. The existence of a single broad peak in the SANS profile strongly suggested the formation of PVME-b-DPS block copolymer. Although the block copolymer was totally in the disordered state within the temperature range between 34 and 206 degrees C, an increase in the scattering intensity with temperature implying an increase in the amplitude of thermal concentration fluctuation was clearly observed, as expected from the LCST-type phase behavior of PVME/PS blends.