An investigation has been conducted into the effects of photodegradation on the crystallinity and melting behaviour of isotactic polypropylene (PP). PP samples having different structural characteristics were prepared and exposed to ultraviolet radiation (u.v.) in the laboratory for periods of up to 48 weeks. The changes in crystallinity during exposure were followed by X-ray diffraction and differential scanning calorimetry (d.s.c.), whereas the chemical degradation of the specimens sampled was evaluated by gel permeation chromatography (g.p.c.) and Fourier transform infrared spectroscopy (f.t.i.r.). An increase in fractional crystallinity during u.v. exposure was noted for all types of samples studied, and the gain in crystallinity was usually between 6% and 7% and was virtually independent of the initial structure of the polymer. Measurements conducted at different depths within the test bars indicated that the fractional crystallinity increased during u.v. exposure due to crystal growth using molecule segments released by the scission of molecules (probably taut molecules). This process, called chemi-crystallization, is restricted by the chemical defects introduced into the molecules by the photodegradation. Possible mechanisms for the chemi-crystallization process are discussed. The melting thermograms of most types of samples exhibited single peaks with melting range increasing with exposure time. The broadening of the melting thermograms occurred during the period between the commencement and the completion of the chemicrystallization. Secondary crystallization proceeded much more slowly in the interior of the test bars because of the limited oxygen supply, but the final crystallinity was the same as the plateau value, which was attained more rapidly when the material in the interior was exposed by machining away the surface prior to u.v. exposure.