A series of poly(ethylene adipates) with molecular weights between 1800 and 15 000 were prepared, and were extended by reaction with di-isocyanates. In one series of extended polymers the different polyesters were all extended with tolylene di-isocyanate whilst in a second series a polyester of molecular weight 3000 was extended with 4 different di-isocyanates. The glass transition temperatures and melting points of all polymers were studied. The variation of Tg of homopolymers with molecular weight was not adequately represented by the usual equations and it was concluded that the OH end groups are tied into the structure. On the other hand the variation of T,, with molecular weight obeyed the Flory equation and leads to a value for the heat of fusion similar to that obtained by Edgar and Ellery by the copolymer method. The observed values of Tg and Tr, of the copolymers did not agree with values calculated by random copolymer theory. In the case of melting points it is likely that the discrepancy arose from the disparate nature of the ethylene adipate, crystallizable, units and the di-isocyanate, non-crystallizable, units; the latter distorted the structure so badly that a number of neighbouring crystallizable units were prevented from crystallizing.