A process for forming extended chain polyesters and block or graft polyesters or copolyesters which comprises the steps of (a) forming an intimate mixture of one or more polyesters, and an effective amount of one or more chain extension reagents, which are phosphorous (III)-containing compounds, and (b) heating said mixture for a time and at a temperature sufficient to form said extended chain, or graft or block copolyester or polyester; wherein the polyester used in step (a) contains no or substantially no substances which comprise elements from groups VA and VIA of the periodic table in elemental form; and wherein preferably, said polyesters conatain one or more elements belonging to groups IIA, IIIA, IVA, VB, IIB, IIIB, IVB, VIII and the lanthanides.