A variety of inorganic aluminium sols were investigated for their reactivity with colloidal silica to form polycrystalline mullite. Nuclear magnetic resonance (NMR) spectroscopy was used on the precursor aluminium sols to ascertain the co-ordination and reactivity of the species present. Differential thermal and thermogravimetric analysis and X-ray diffraction were used to analyse the products of the fired xerogels. Commercial reactants produced diphasic materials with little or no mullite formed at 1000 degrees C. A monomeric/dimeric sol produced in house however, showed a sharp exotherm at 920 degrees C and crystalline mullite formation at this temperature.