Abstract: Modified polyethylene terephthalate polymers, particularly suitable for making carbonated drinks or mineral water bottles by injection (stretch) blow moulding, include a small proportion (from 0.007 to 0.08 mole %) of residues of a polyfunctional hydroxyl compound such as pentaerythritol as a chain branching agent and have (after solid phase polymerisation processing) melt viscosity values of from 300 to 1050 Pa.s (3000 to 10500 Poise) at 295.degree. C. (approximately Intrinsic Viscosity 0.6 to 0.9). The polymers also preferably include from 0.5 to 5 mole % of residues derived from a chain orientation disrupting monomer such as iso-phtalic acid. Best results are obtained in restricted ranges of polymer viscosity, hydroxyl compound and also chain orientation disrupting monomer level. As compared with conventional PET the polymers have the advantage that solid phase polymerisation rates are much improved.