Photooxidation of 4,4＇-methylene bis(4-phenylisocyanate) and toluene diisocyanate based poly(ester-urethanes) has been followed by means of Fourier transform infrared spectroscopy as a function of soft segment molecular weight. The results have been related to the hard/soft segment phase mixing, studied by differential scanning colorimetry. The extent of the photolysis reaction in the urethane linkage is more relevant in higher soft segment molecular weight based polymers. The phase separated structure is a critical factor which must be taken into account in order to study the extent of the photodegradation of the hard segments in aromatic poly(ester-urethanes). (C) 2000 Elsevier Science Ltd. All rights reserved.