Surface-modified CdS nanoparticles selectively dispersed in hexagonally packed poly(ethylene oxide) (PEO) cylinders of poly(styrene-b-ethylene oxide) (PSEO) block copolymers were prepared. The photoluminescence and ultraviolet-visible characteristics of the presynthesized CdS nanoparticles in N,N-dimethylformamide and in PEO domains of the PSEO block copolymers were determined. Because of strong interactions between the CdS nanoparticles and PEO chains, as shown by Fourier transform infrared spectroscopy, the incorporation of the CdS nanoparticles prevented the PEO cylinders from properly crystallizing; this was confirmed by differential scanning calorimetry and wide-angle X-ray diffraction measurements. The intercylinder distance between the swollen and reduced-crystallinity CdS/PEO cylinders in turn increased, as confirmed by small-angle X-ray scattering and transmission electron microscopy. At a high CdS concentration (43 wt % or 8.3 vol % with respect to PEO), however, the hexagonally packed cylindrical nanostructure of the PSEO diblock copolymers was destroyed. (c) 2005 Wiley Periodicals, Inc.