Composite microgels with polystyrene nano particles were synthesized by seeded emulsion polymerization of styrene in the presence of pH- and temperature-responsive poly(N-isopropylacrylamide-co-methacrylic acid) microgels as seeds. In particular, the core microgels maintained their swelled state as the pH was increased to 10 during seeded emulsion polymerization conducted at an elevated temperature. Furthermore, we tuned the swelling degree of the core microgels at pH 10 by changing the amount of methacrylic acid incorporated during the synthesis of the core microgels. Unlike deswollen microgels, during the seeded emulsion polymerization, the swollen microgels were covered with a monolayer of non-close-packed polystyrene particles on their surface, as confirmed by electron microscopy. A possible mechanism for the seeded emulsion polymerization of styrene in the presence of swollen microgels under alkaline conditions is proposed.