Simple mixing of powders does not always result in sound composite materials with designed properties after sintering owing to the inherent solid solution of constituent phases at high temperatures. A modified sintering process, where the relative particle sizes of constituent phases are controlled to minimize interdiffusion between starting materials, has been proposed to achieve the formation of a composite having tailored physical properties. A 0.5 Al2O3-0.5 AlN ceramic composite which was fabricated by the proposed powder process showed design-based thermal and electrical conductivities following Rayleigh's derivation, contrary to conventional Al2O3/AlN, which undergoes detrimental interdiffusion.