The formation and growth characteristics of Fe based aluminide diffusion layers at the Fe-Al interface have been analysed in terms of interfacial interaction potentials based on the statistico-thermodynamical theory of multicomponent alloys combined with electronic theory in the pseudopotential approximation. The pairwise interatomic interaction potentials and partial ordering energies have been calculated to predict the effect of various alloying additions on the activity coefficient of Al atoms in alpha-Fe-(0.95)(Al(1-n)X(n))(0.05) alloys where X = Si, Ti, Ge, Sb, Mg, Cu, Ca, Ag, Cd, Cr, Co, Zn, Mn, Ni, Pb or Bi and considered up to 1 at.%. The results of calculation show that the impurity elements with regard to their effect on the activity coefficient of Al atoms may be classified into two groups; I-group impurity elements, X(I)=Si, Ti, Ge, Sb, Mg, Cu, Ca, Ag, Cd, and Cr which decrease the activity coefficient of Al atoms, reduce the thickness of the Fe-Al intermetallic diffusion layer and II-group impurity elements, X(II) = Co, Zn, Mn, Ni, Pb and Bi, which increase the activity coefficient of Al atoms, tend to increase the thickness of diffusion layer at the Fe-Al interface. It has been established, in agreement with experimental observation, that the value of activity coefficient of Al atoms in alpha-Fe(0.95)(Al(1-n)X(n))(0.05) alloys has a strong influence on the formation and growth kinetics of interfacial diffusion layers at the Fe-AL interface. The analysis of the effects of type and content of X(I) and X(II) impurity elements on the activity coefficients of Al gamma(Al)/gamma(Al)(0), in alpha-Fe(0.95)(Al(1-n)X(n))(0.05) alloys is in good qualitative agreement with the experimental results reported in the literature. (C) 1998 Acta Metallurgica Inc.