- A first-principle procedure is applied for a priori estimation of the efficiency of different additives to prevent fiber degradation in metal-matrix composites. The procedure is based on the pseudopotential calculations of excess energies of interstitial solid solutions, while the parameters of model pseudopotentials are obtained from band structure calculations, executed by the linear muffin-tin orbitals method. The aluminium-matrix/carbon-fiber composite is considered as an example with Fe, Zn, Mg, and Si additives to the matrix. The limits of solubility of carbon in aluminium with and without additives are evaluated. The calculation was checked experimentally for the system C(Al, Si). The results confirm the expectations of the calculation.