- The problem of stability of interfaces in metal matrix composites is reduced to the investigation of the solubility and diffusion processes of fiber atoms in the metal matrix. The calculations of excess thermodynamic functions of interstitial solid solutions is performed. The influence of additives on the stability of intermetallic phases formed at the interfaces is discussed. The height of diffusion barriers for aluminium and copper matrix composite materials with carbon or boron fibers is calculated. It is shown that the alloying of the matrix by additional element, substituting aluminium or copper in the interstitial metal-metalloid solid solution, changes the value of the barrier and influence the diffusion of metalloid. Some additives as the best alloying elements for preventing the diffusion in the alloys studied are predicted.