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Ultrafine powders of aluminum‐rich mullite precursor phases were prepared by spraying mixed acid solutions of silicic acid and aluminum chloride into ammonium hydroxide at different pH values. The aluminum‐rich composition was chosen to avoid abnormal grain growth of the mullite grains caused by silica‐rich liquid phases. The influence of coprecipitation pH on particle size, zeta potential, and the mixing degree of aluminum and silicon in the precipitated powders and on the sintered microstructures have been described. In the basic pH region, the degree of mixing of both components increased as the pH of coprecipitation decreased. Under these conditions, direct slip casting of the coprecipitated slurries was possible, and fine and equiaxed microstructures could be obtained in the sintered bodies.