A kinetic analysis was performed on the spinel (MgAl2O4) formation reaction at the interfacial region of the calcium aluminate (CaO-Al2O3) fibre-reinforced aluminum 7075 alloy matrix composites (CA/Al 7075) processed at 1200 degrees K. The counterdiffusion model of Mg2+ and Al3+ fluxes was employed for the spinel formation reaction. From the parabolic relation between the thickness of the spinel layer and time the reaction rate constant was determined to be a function of the average diffusion coefficient of Al-3+ ions. By applying the parabolic equation to the experimental data of the CA/Al 7075 the average diffusion coefficient of Al3+ ions in the MgAl2O4 spinel at 1200 degrees K was calculated and this value appeared to be in small deviation from the extrapolation of previous diffusion coefficient data.