The optimization of fibre/matrix interfaces in Nicalon-fibre-reinforced aluminium-phosphate-matrix composite materials is addressed. First, the structure and chemical composition of the fibre/matrix interfaces were characterized for the as-fabricated composite materials and for the same materials after high-temperature exposures simulating the conditions of their intended application. Transmission electron microscopy (TEM) showed considerable Si diffusion from the fibres into the matrix accompanied by the formation of an interfacial diffusion/reaction zone in the process of heat treatment at 816-degrees-C and higher temperatures. A BN-fibre coating did not prevent Si diffusion. Next, the interfacial bond strength was measured for the uncoated and some of the coated interfaces. The measurements showed a much lower bond strength in the carbon and carbon//BN coated interfaces than in the uncoated and BN-coated interfaces. Finite-element modelling was used to evaluate the interfacial bond strength which would result in the highest strength of the composite material. This optimal bond strength was found to be characterized by a critical energy-release rate close to 50 J m-2. Further increase in the interfacial bond strength above 50 J m-2 resulted in brittle failure of the composite materials. Finally, the potential fibre coatings which were stable and not reactive with the fibres and the matrix at elevated temperatures were identified for the projected service temperatures of 816 and 1093-degrees-C.