The phase behaviours in binary blends of styrene-isoprene diblock copolymer and poly(vinyl methyl ether) (PVME) were investigated by using small-angle X-ray and light scattering techniques. It was observed that the added homopolymer molecules were dissolved into the microdomains of the styrene blocks of the copolymer. The blends exhibited a lower critical solution temperature as seen in the blends of PVME with polystyrene. The experimental results were compared with that of the modified Meier’s theory in a semiquantitative way.