Abstract: A fast crystallizing polyester wherein the polyester has an intrinsic viscosity of greater than 0.3 and at least 90 mole % of the repeating units of the polyester are residues obtained from aromatic diacids or hydroxyacids and aliphatic diols characterized in that at least some of the acid end groups of the polyester have the formula --Y.sup.- M.sup.+ where M is an alkaline metal ion and Y is an anion selected from carboxylate, sulphinate, phosphinate, phosphate, sulphonate, phosphite and borate ions, which ionized end groups have been produced during polymerization of the polyester by either:(a) copolymerizing units derived from materials of formula X--R--Y.sup.- M.sup.+ wherein X is a group capable of being polymerized onto the polyester chain, R is an aliphatic, cycloaliphatic or aromatic group, or(b) polymerizing the polyester in the presence of an alkaline metal salt or corresponding base,the concentration of groups of formula Y.sup.- M.sup.