The crystal-crystal transition of isotactic polybutene-1 (iPB-1) from form II to I at room temperature is investigated with in-situ Fourier transform infrared spectroscopy (FTIR), Fourier transformation infrared microspectroscopic imaging (FTIRI) and synchrotron radiation scanning X-ray micro-diffraction (SR-mu SXRD). The transition rate from form II to I shows a non-monotonic correlation with crystallization temperature on samples isothermally crystallized at different single temperatures. An abnormal spatial distribution of transition rate is observed in samples prepared with a two-step crystallization approach, in which samples crystallized at high temperature and then were quenched to low temperatures for further crystallization. A maximum transition rate occurs around the edge of large spherulites formed at high temperature, which cannot be interpreted by the effect of crystallization temperature alone. The accelerated transition rate in this region is attributed to internal stress, where an intermediate state, revealed with SR-mu SXRD, may be the structural origin for the fast transition rate from form II to I. (C) 2013 Elsevier Ltd. All rights reserved.