The morphology of segmented copolyether-esters with low hard segment content has been studied by TEM. It has been found that the size and shape of these domain structures are determined by two competitive factors: the rate of phase separation of the components and the rate of crystal growth. The fine struture in domain is related to the nucleating rate

and crystal growth rate. In case of samples with PET as hard segment the ratios of Va/Vc and Va/Vn are larger than those of samples with PBT as hard segment.