The Microstructure of quenched and annealed IPP films was investigated by means of differential scanning calorimetry (DSC), density measurements, wide-angle X-ray diffraction (WAXD) and dynamic mechanical spectrometry (DMS). It was found that quenched IPP can be described as a biphasic material constituted of an amorphous phase strongly cross-linked by many crystalline entities exhibiting both small size and very low degree of perfection. Such microcrystallites act as true physical ties of the amorphous phase. On increasing the annealing temperature from 20 degrees C (quenched film) to 160 degrees C, the crystallinity ratio first remained constant for annealing temperatures between 20 and 93 degrees C and then it increased. Subsequently, both size and degree of perfection of crystalline entities progressively increased and tended towards the characteristics of the monoclinic phase. This resulted in a progressive decrease in the physical cross-linking degree of the amorphous phase, even for the samples exhibiting the highest crystallinity ratio.