Two random propylene copolymers with low ethylene content synthesized by Ziegler-Natta catalysts were used is this study to investigate the formation of gamma-crystal phase during isothermal crystallization at high pressures. At atmospheric pressure these copolymers crystallize in a mixture of alpha- and gamma-crystals. The content of the gamma-phase in the copolymer crystals increased with increasing defect content, crystallization temperature and pressure. Wide-angle X-ray diffraction studies showed that crystallization of these copolymers at pressures above 88 MPa and temperature above 142 degrees C leads to formation of pure gamma-phase. The equilibrium melting temperature of the gamma-phase has been determined as a function of defect content and crystallization pressure. Temperature-pressure-composition alpha-gamma phase diagram of isotactic polypropylene was constructed based on the Gibbs free energy approach. This diagram enabled the extrapolation of the equilibrium melting temperatures of both phases for defect free isotactic polypropylene. They were found to be 186.9 degrees C for the alpha-phase and 189.9 degrees C for the gamma-phase. (c) 2006 Elsevier Ltd. All rights reserved.