The influence of pH on the incorporation of a functional monomer (itaconic acid or methacrylic acid) on core-shell polymers of styrene-butyl acrylate and on their mechanical properties is investigated. The structured polymers were prepared by a two-stage emulsion polymerization process. It was found that the composition of the shell is strongly affected by the pH of the reacting medium. The amount of functional monomer that is incorporated in the shell increases as the pH decreases and becomes a maximum when the pH falls below the pK(a) of the functional monomer. The mechanical properties (ultimate elongation, ultimate strength, and toughness) improve as the amount of incorporated functional monomer increases.