Nanosized mullite matrix precursors comprising individually a mixture of a boehmite sol (Remal A20) and either a hexagonal colloidal silica sal (Akzo-Nobel, Bindzil 30/220) ol one of a series of Aerosil amorphous fumed silica powders in aqueous suspension, have been characterised in terms of particle size, particle size ratio, rheological behaviour; and solids loading. Particle mobility measurements and TEM were employed to determine the heterocoaguluted particles' net surface charge and the nanoscale alumina-silica particle interaction behaviour respectively. The suspension finally chosen for further investigation was optimised to obtain a heterocoagulated mullite precursor sol suitable Soi composite fabrication by electrophoretic filtration deposition. This technique, in conjunction with pressure filtration, is uf major technological relevance to the fabrication of ceramic matrix-composures; for ex-ample, the fabrication of multilayer alumina fibre reinforced mullite composites with nanosized matrix grains. It has been shown that nit aqueous colloidal suspension, comprising two different nanosized particle species (i.e. boehmite and silica) and exhibiting a solids loading as high as 30 wt-% of heterocoagulated boehmite and silica particles ui?der stable dispersion conditions, carl be prepared if the short, range particle-particle interactions are well controlled via homogeneous hetero-coagulation of the constituent boehmite and silica coagulation of the particles. (C) 1998 The Institute of Materials.