Small-angle X-ray scattering (SAXS) profiles have been measured for blends of one-end-aminated polystyrene (APS) and one-end-carboxylated or -sulfonated poly(ethylene glycol) (CPEG or SPEG) with weight fraction of 50/50 per cent. In the profiles of blend solutions in toluene (TL), APS/CPEG/TL and APS/SPEG/TL with solvent weight fraction of 0.2 at 80 degrees C, there exists a scattering maximum around a Bragg spacing of 203 Angstrom for the system containing SPEG, whereas there is no peak for the system containing CPEG. Similarly, the scattering peak is absent in the profile of the blend APS/CPEG without solvent, and is observed in that of APS/SPEG at 80 degrees C. The APS/SPEG blend sample cooled to a temperature below the melting point of SPEG shows a more intense SAXS peak than that at higher temperature. The similarly cooled blend of APS/CPEG exhibits a weak scattering maximum, and its position is nearly identical to that of pure CPEG. From these observations and previous studies on the phase diagram of blend solutions in TL, it has been suggested that the APS/SPEG blend behaves like a diblock copolymer owing to the strong association between the terminal amino and sulfonic acid groups, and an ordered structure that is similar to those observed in diblock copolymers is formed even in bulk melt and concentrated solution. Because of the weak association between amino and carboxyl groups, the APS/CPEG system behaves like an ordinary polymer blend, and crystallization of the CPEG chain may occur in a similar manner to the homopolymer in the macroscopically separated domain.