Polymer particles or beads for use as opacifiers in paints are described in which a core is provided with a hard sheath and the particle has one or more voids produced by reaction with a non-­volatile permanent or fixed base. The sheath is a copolymer of an ethylenically unsaturated aliphatic monomer free of acid groups and at least 2 per cent by weight of a soft ester-containing monomer and optionally styrene in an amount of up to 25 mole per cent. The core is formed from an ethylenically unsaturated monomer with acid functionality. The sheath polymer has a glass transition temperature of at least 40°C, The particles have a size of up to 40000 nanometers before swelling the core by some 150 to 2000 per cent.