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Morphological change of a micelle of poly(styrene)-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-PVP-PEO) polymer was induced by binding sodium dodecyl sulfate (SDS) to the PVP block in acidic aqueous solutions. The change in the size of SDS/PS-PVP-PEO complexes was detected by dynamic light scattering measurements and atomic force microscopy, and the binding of SDS was confirmed by zeta-potential measurements. When the micelle was free from SDS in acidic aqueous solutions, the hydrodynamic diameter of the micelle was 216 nm, reflecting the extended conformation of the PVP block due to the repulsion between protonated pyridine units. As the cationic PVP block was electrically neutralized with anionic SDS, the diameter was gradually reduced concomitant with the decrease in zeta-potential and finally reached 175 nm when the PVP block was completely neutralized. The decrease in the diameter shows the morphological change of the PVP block from extended to shrunken forms. Further addition of SDS did not cause the changes of the diameter nor zeta-potential. This indicates that SDS was not bound to the PS-PVP-PEO polymer after the PVP block was fully neutralized and that the hydrophobic binding of SDS to the polymer was negligible due to the low concentration of SDS.