Aluminizing of nickel alloy 718 was studied in order to reveal the effect of combined alloy additions on aluminide growth kinetics, as opposed to pure metal substrates. The low activity pack process was used and treatment was carried out at 1273 K using ammonium fluoride activator and Ni-50 wt% Al powder as an aluminium source for treatment times of 1, 2, 4 and 8 h. The aluminide coatings varied between 40 and 110 mu m in thickness. The microstructures consisted of a NiAl phase with a fine grain size containing small secondary-phase particles at the grain boundaries. The interface between the coating and the substrate was lined by a lamellar layer exhibiting a two-phase structure, which was enriched in chromium and niobium in addition to containing iron and nickel. Weight gain measurements indicated parabolic growth up to 2 h, beyond which the growth rate slowed down. Microstructures and composition profiles revealed that the interlayer, which was enriched in elements insoluble in NiAl, posed a barrier to interdiffusion of the reacting species and slowed down the growth kinetics of the aluminide.