A bi-velocity method is used to simulate the element-concentration evolution in the diffusion zone during the aluminization of a Rene-80 nickel-based superalloy. The basis of the method and the detailed calculations are presented. The model enables the prediction of the formation of intermetallic phases and multi-phase layers. The entropy-production rate can also be calculated, and it is shown that it might serve as a basis to indicate the interphases. The proposed model is applied to model the diffusion in a ternary Al-Cr-Ni system. The results of the calculations are compared with experimental element-concentration profiles, measured for the Rene-80 alloy subjected to aluminization by CVD.