Fine particles having poly(p-styrenesulfonic acid) (polySS) in the core and a polystyrene (polySt) shell were synthesized by cross-linking and successive chemical transformation of triblock copolymer micelles. A triblock copolymer, polySt-b-poly(St(0.8)-co-SBS0.2)-b-polySSPen, was synthesized by nitroxide-mediated living radical polymerization, where SBS and SSPen denote p-(1-methylsilacyclobutyl)-styrene and neopentyl p-styrensulfonate, respectively. Block copolymer micelles carrying a polySSPen core and polySt shell with cross-linkable silacyclobutyl groups in the shell were prepared by dissolving the triblock copolymer in cyclohexane. Cross-linking of the shell by Pt-catalyzed ring-opening of silacyclobutane followed by thermolysis of the neopentyl esters in polySSPen in tetralin in the presence of p-toluenesulfonic acid gave the cross-linked micellar particles with polySS in the micelle core. The obtained particles showed unique environmentally responsive solubility due to the core-shell structure with a moderately cross-linked shell.