An improved method of forming a composite body of a metal, intermetallic or ceramic matrix reinforced with niobium filaments, particles, platelets or mixtures thereof, the method comprising admixing the niobium reinforcing material with powders of the matrix component elements, forming the admixture into a desired shape and converting the powders to a matrix reinforced with the niobium material, the improvement wherein the reinforcing material has a surface coating thereon of a compound Nb.sub.2 O.sub.5, wherein the compound NbO reacts during formation of the matrix with a portion of at least one of the powdered elements to form a barrier layer at the reinforcer-matrix interface to prevent further reaction between the reinforcer and the matrix component elements. Also disclosed is a method of treating niobium particles, filaments, platelets or mixtures thereof by exposing the surface thereof to molecular O.sub.2 at temperatures and pressure conditions such that the niobium and molecular O.sub.2 react to form a surface coating on the niobium material of the compound Nb.sub.2 O.sub.5. Also disclosed are the novel products of the above-described methods.