Structural ordering from the al to the alpha2 phase of isotactic polypropylene has been investigated. The DSC studies showed that reorganization or recrystallization occurred in the wide endothermic region. In the high-temperature X-ray experiment of the sample annealed at 135 degreesC, the (2) over bar 31 and (1) over bar 61 reflections ascribed to the alpha2 phase began to increase from 160 degreesC and to decrease above 172 degreesC. The b-axis length and the angle beta of the unit cell increased up to 155 degreesC and decreased afterward. The latter decrease was well correlated with an increase in the alpha2 phase. The alpha2 fraction increased in the temperature region where the degree of overall crystallinity decreased. Reorganization or recrystallization into the a2 phase is thus considered to occur with the fusion of the al phase, and the growth of the alpha2 phase in the solid state seems to be difficult. (C) 2001 Elsevier Science Ltd. All rights reserved.