Grain growth kinetics of mullite in laboratory-produced and commercial (3M-Nextel 720) alumino silicate fibers was analyzed in the temperature range between 1500 degrees and 1700 degrees C. The lab fibers consist of mullite plus traces of alpha-alumina, while the phase content of the commercial Nextel 720 fibers is about 60 wt% mullite plus 40 wt% alpha-alumina. The temperature-induced rain coarsening of mullite follows the empirical law D-1/n-D-0(1/n) = kt. Two different temperature regimes can be distinguished with respect to the grain growth exponents: above 1600 degrees C the growth exponent is approximate to 1/3, while below 1600 degrees C the growth exponent of approximate to 1/12 is exceptionally low. Laboratory-produced and commercial fibers show almost identical mullite grain growth kinetics.