The phase structure of crystalline isotactic polystyrene (iPS) has been investigated with temperature-modulated differential scanning calorimetry (TMDSC), wide-angle X-ray scattering (WAXS), and Fourier transform infrared (FTIR) spectroscopy. Quenched amorphous samples have been cold-crystallized at 140 or 170degreesC for various crystallization times. The degree of crystallinity obtained from WAXS, with the ratio of the crystal peak intensity to the total peak intensity, shows excellent agreement with the crystallinity determined from TMDSC total heat flow endotherms. For the first time, FTIR results show that the absorbance peak ratio (I-981cm(-1)/I-1026cm(-1)) has a linear correlation with the crystalline mass fraction (chi(c)) for cold-crystallized iPS according to the following relation: I-981cm(-1)/I-1026cm(-1) = 0.54chi(c) + 0.16. This relationship allows the crystallinity of iPS to be determined from infrared spectroscopy analyses in cases in which it is difficult to perform thermal or X-ray measurements. On the basis of the measurements of the heat capacity increment at the glass transition, we find that a significant amount of the rigid amorphous fraction (RAF) coexists with the crystalline and mobile amorphous phases in cold-crystallized iPS. The RAF increases systematically with the crystallization time, and a greater amount is formed at a lower crystallization temperature. A three-phase model (crystalline phase, mobile amorphous phase, and rigid amorphous phase) is, therefore, appropriate for the interpretation of the structure of cold-crystallized iPS. The origin of the low-temperature endothermic peak (annealing peak) has been investigated with TMDSC and FTIR spectroscopy and has been shown to be due to irreversible relaxation of the RAF. (C) 2003 Wiley Periodicals, Inc.