We studied phase behavior and molecular assembly in blends of poly(vinyl methyl ether) (PVME) and polystyrene-bloch-polyisoprene (SI) by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and light scattering. The neat block copolymer forms bcc (body centered cubic) spheres composed of polyisoprene block chains in a matrix of polystyrene block chains. On casting from a toluene solution, the blends with volume fractions of PVME less than or equal to 0.3 experienced only microphase separation, resulting in a structure with spherical microdomains of polyisoprene block chains (PI) in a matrix of polystyrene block (PS) chains (PS-corona) mixed with PVME, while the blends with volume fractions of PVME greater than 0.3 underwent macrophase separation into a PVME-rich region and an SI-rich region which was also microphase-separated. Furthermore, a thermoreversible one- to-two-phase transition induced by the change of segmental interactions between PS-corona and PVME was observed for the cast films. This system can be considered as a two-component system composed of nanocolloidal spheres of PI with PS-corona and PVME with a LCST phase diagram. The cloud point of the nanocolloidal spheres/PVME system is significantly lowered compared to that of a homopolystyrene/PVME blend due to the confinement of PS block chains onto the PI spheres. Note that the structures observed for this system are similar to those observed in colloidal dispersions of ionic latex particles in water and oil-in-water or water-in-oil microemulsions.