The order-disorder of the tetrahedrally coordinated aluminum and silicon atoms in mullite has been investigated by means of Si-29 nuclear magnetic resonance (NMR) spectroscopy. Sinter (3/2) and fused (2/1) mullites in the as-received state and reheated at 1750 degrees C, and a reference sillimanite were used for this study. All mullites display similar Si-29 NMR spectra: The strongest peak occurs at about -88 ppm, with two subpeaks close to -92 and -96 ppm. The -88 ppm signal is assigned to a sillimanite-type environment with three aluminum oxygen tetrahedra as next nearest neighbors of the silicon oxygen tetrahedra. The two Si-29 NMR signals near -92 and -96 ppm are assigned to silicon oxygen tetrahedra surrounded by two aluminum oxygen and one silicon oxygen tetrahedra, and one aluminum oxygen and two silicon oxygen tetrahedra, respectively. Si-29 NMR spectra with different short-range-order parameters were simulated by an array of 2 x 10 000 tetrahedral positions by means of an adapted random generator. The comparison between measured and simulated mullite and sillimanite Si-29 NMR spectra yields a moderate degree of tetrahedral aluminum-silicon order, with no tendency toward cation demixing.