Tetraethoxysilane (TEOS) and Al-sec-butylate (Al-O-Bu) were used for the sol gel synthesis of mullite ceramics. The starting materials had bulk compositions corresponding to values between 72 and 78 wt % Al2O3, and 28 and 22 wt % SiO2, respectively, and were calcined at 400-degrees-C (A-series) and 1100-degrees-C (B-series). B-series samples, despite their higher green densities, could only be sintered to about 65-70% TD (theoretical density) at 1650-degrees-C, whereas A-series samples achieve values of about 93-98% TD. Ceramics with relatively high amounts of glass phase from large tabular mullite crystals, which are embedded in a finer-grained mullite matrix. As soon as the bulk Al2O3 content increases, equiaxed mullite grains appear and the mean grain size becomes smaller, showing a significant difference between the nucleation and crystal growth mechanisms of mullites formed in samples with the lower and higher Al2O3-bulk compositions. Depending on the bulk composition of the samples, the temperature-controlled solid-solution of mullite ranges between about 72.7 and 74.3 wt% Al2O3 at 1600-degrees-C and 74.1 and 75.4 wt% Al2O3 at 1800-degrees-C, indicating that the solid-solution region bends over towards the Al2O3-side of the Al2O3-SiO2 phase diagram.