The present invention concerns a composition being an inverse emulsion comprising cross-linked anionic, organic polymeric microparticles, said microparticles having an unswollen z-average particle size diameter of less than 0.75 μm, wherein the organic polymeric microparticles are formed from a monomer mixture comprising (a) an ethylenically unsaturated carboxylic acid, or a salt thereof; (b) an ethylenically unsaturated non-ionic monomer; and (c) from 1 to 10 mole % of 2-acrylamido-2-methylpropanesulphonic acid, or a salt thereof. The invention also provides a process for preparing the composition, the use of the composition in a paper or paperboard making process and methods of making paper or paperboard employing said composition.