The time-resolved measurement of wide-angle X-ray scattering was performed with a synchrotron radiation source during the processes of the isothermal crystallization and ferroelectric phase transition of a vinylidene fluoride/trifluoroethylene copolymer with 73 mol % vinylidene fluoride. When the sample was cooled rapidly from the melt to the temperature region of the paraelectric high-temperature phase, the peak position of the 200/110 reflection shifted toward the higher angle side and the half-width became narrower. This indicated an increase in the crystallite size with a more compact chain-packing mode. Even when the temperature jump was made from the melt into the region of the ferroelectric or low-temperature phase, the crystallization of the high-temperature phase was first observed before the appearance of the low-temperature phase. This was consistent with a prediction based on the so-called Ostwald state rule: the thermodynamically unstable but kinetically preferable high-temperature phase can appear first even when the thermodynamically more stable loco-temperature phase should be created. The time-dependent intensity changes were analyzed with the Avrami kinetic equation. (C) 2004 Wiley Periodicals, Inc.