Isothermal crystallization of a high molecular weight PEO (Mw=4,000,000) has been investigated using photoDSC. Combining light irradiation, heating and DSC analysis, photoDSC demonstrates good capability to follow the in situ photo- and thermo-ageing of semi-crystalline polymers. Isothermal crystallization of PEO was performed at 55 degrees C. After ageing at different temperatures ranging from 0 to 90 degrees C and for various periods of time, the kinetics of this crystallization was found following the Avrami theory. Avrami exponent, n, was found between 0.9 and 1.4 evidencing a one-dimension growth process. It was also found that the isothermal crystallization rate (i.e. reciprocal crystallization half-time) was exposure time, ageing temperature and light intensity dependent which makes this kinetic parameter a good indicator of following and comparing the degradation of the semi-crystalline polymers.