<p>A process for the manufacture of core/shell polymer particles comprises forming a core particle containing acid functionality, a first shell around said core of a copolymer of a nonionic monoethylenically unsaturated aromatic monomer and a copolymerisable polar monomer in an amount of at least 15% by weight of the copolymer and having a solubility in water at 20 DEG C of at least 1% by weight and a second shell around said first shell of a polymer or copolymer of a nonionic monomer having a solubility in water at 20 DEG C of less than 1% by weight and swelling the particle with a non-volatile alkali to generate one or more vesicles. Preferably each shell contains a polymer or copolymer of styrene.</p>
