Cordierite-mullite composites were fabricated from various starting powder mixtures to study the effect of different mixing combinations of the starting materials (commercial oxide sols and/or hydroxide powders) on the resultant mullite grains in the sintered composites. The morphology of the mullite grains was substantially changed depending on the Al-source materials used. Acicular grains were produced from the alumina sol-containing mixtures, whereas the growth of angular or granular mullite grains occurred for the mixtures containing Al(OH)3 powder. When using alumina sol, its higher reactivity combined with better dispersability in precursor powders could cause the lowering in the mullitization temperature than for those containing Al(OH)3 powder. In addition, it could produce very fine mullite seed particles in a calcined powder, leading to the growth of the acicular mullite grains in the sintered C/M composites.