Abstract: The production is described of a dispersion of polymer microparticles, which may optionally be crosslinked, by dispersion polymerisation in aliphatic hydrocarbon liquid under specified conditions of a monomer mixture which comprises up to 50% of styrene or a substituted derivative thereof and up to 50% by weight of a derivative of maleic acid or fumaric acid which is soluble in the hydrocarbon liquid at the polymerization temperature. The microparticles have a size in the range 100-400 nm and a high refractive index, and are suitable for incorporation in coating compositions which are based on a film-forming polymer, such as a thermosetting acrylic resin or an alkyd resin, containing a significant proportion of aromatic residues, and which are intended to be used either unpigmented or when containing only light-absorbing pigments.