Films of isotactic polypropylene (iPP) were quenched to between 5° and 40°C, with a very thin substrate. Wideangle x-ray diffractograms for these samples show that above 25°C the monoclinic phase appears, in addition to the smectic. The amorphous fraction was derived from the study of transport properties; it was the same for all the samples and different from the value obtained from density data. It is suggested that at room temperature, just before the monoclinic phase appears, there is a complex structural reorganization, in which the smectic phase contains many voids and imperfections. This makes the use of density data unapplicable for determination of the phase composition. The variation of transport properties was studied during the aging of the sample quenched at 25°C. From this study it is suggested that the aging phenomenon occurs in two stages: a rapid decrease of the amorphous fraction followed by a progressive reduction of mobility of the amorphous chains.