A theoretical model which combines gaseous transport and solid state diffusion was used to study aluminide coating process by pack cementation. The aluminide coatings were applied in the high activity pack containing NH₄Cl activator with Ni substrate under argon atmosphere. On the basis of the process conditions, the suggested model allows the surface composition, the growth rate of coating layers and the aluminium concentration profiles in coatings to be calculated. In the case of NH₄Cl activator, careful consideration was required in the analysis, because activator contains nitrogen and hydrogen as well as halogen element to activate the pack. A good agreement is obtained between the theoretical predictions and the experimental results.