High-resolution synchrotron powder X-ray diffraction (XRD) experiments were conducted to clarify the transformation of sillimanite to mullite (mullitization) and determine the mullitization temperature (T-c). We were able to distinguish sillimanite and mullite in the XRD patterns, despite their very similar crystallographic parameters, and to detect the appearance of small mullite peaks among sillimanite peaks. Analysis of the Johnson-Mehl-Avrami (JMA) equation for mullitization ratio (zeta) revealed that at temperatures T >= 1240 degrees C the mullitization had the same kinetics. The activation energy E at T >= 1240 degrees C obtained from the Arrhenius plot was 679.8 kJ mol(-1). In analysis using a time-temperature-transformation diagram for mullitization, a mullitization curve of zeta= 1% can be described as In t = (1/n)In In(1/1-zeta) - In k(0) + A/ [RT(T-Tc)(2)] + E-A]RT where t is time, n is a reaction-mechanism-dependent parameter determined as 0.324 by JMA-analysis, k(0) is the frequency factor, EA is the activation energy for atomic diffusion, and Al(T-c - T)(2) represents the activation energy for nucleation. The results of fitting the data to this equation were T-c= 1199 degrees C, A = 3.9x10(6) kJ mol(-1) K-2, E-A = 605 kJ morl, and k(0) = 3.65x10(15). We conclude that the boundary between sillimanite and mullite+SiO2 in the phase diagram is similar to 1200 degrees C.