Solubilization behaviour of poly(ethylene oxide)s (PEOs) (MW = 200-1000) into a rod-coil polymer of ethyl 4-[4’-oxy-4-biphenylcarbonyloxy]-4’-biphenylcarboxylate with poly(ethylene oxide) at a degree of polymerization of 12 (BCEO-12) was studied by using small angle X-ray scattering (SAXS). PEOs were partially miscible with the PEO block of the rod-coil copolymer whether the rod-block of BCEO-12 was crystalline or liquid-crystalline. The amount of PEO which could be solubilized into a domain of coil block decreased on increasing the molecular weight of the added PEG. The solubility limit of PEO increased remarkably when the rod block of BCEO-12 was molten to form a smectic-A phase. This indicated that the solubility limit also increased on reducing the grafting density of coil block.