Spherical colloidal particles of mixed silica-alumina composition of narrow size distribution were prepared by cohydrolysis of mixed alkoxides in the presence of sulfate ions. The ratio of the constituents could be altered by the solution pH and the reactant concentrations. Analysis of the composition of growing particles showed that they were internally inhomogeneous, which was substantiated by microelectrophoresis. The solids as prepared were partially crystallized to mullite on calcination at 1100-degrees-C. During this process the powders retained the spherical shape, but underwent some sintering.