Thermal crystallization from the glassy state was investigated for isotactic polystyrene (iPS) at three temperatures (140°C, 170°C, and 200°C) for different times. The crystallized samples were analyzed by x-ray diffraction, differential scanning calorimetry (DSC), and transport properties of dichlo- romethane vapor. X-rays and DSC give very similar values of crystallinity. At variance, sorption measurements indicate a fraction of amorphous phase much lower than the previous. The presence of a mesophase, impermeable to the vapors at low activity, was hypothesized. Its value is constant for annealing at 140°C, whereas it decreases with time at 170°C and 200°C, in correspondence with an increase of the crystalline fraction.