A mullite/mullite nanocomposite powder has been synthesized, composed of nanometer-size 3Al(2)O(3)center dot 2SiO(2) ('3 : 2') mullite precipitates within a matrix of the high alumina 2Al(2)O(3)center dot SiO2 ('2 : 1') mullite. Historically, the transition from the metastable high-alumina phase to the thermodynamically stable '3 : 2' phase of mullite has been thought to be a continuous process, involving a continuous solid solution between the two forms of mullite. In contradiction to this widely held view, our high resolution transmission electron microscopic characterization confirms that a first order phase transition between two distinct mullites occurs. The high degree of interface coherence between the precipitates and the matrix allows us to speculate that the mechanical properties of the matrix could be enhanced by a process similar to the precipitation hardening of metals.