The isothermal oxidation of a NiAl and a NiAlY alloy and of a sputtered NiAl microcrystalline coating were investigated at 1000 degreesC in air by TGA, XRD and SEM, The isothermal oxidation kinetics of both NiAl and NiAlY alloys and NiAl microcrystalline coating followed the parabolic rate law with two subsequent stages. A phase transformation from metastable theta -Al2O3 to stable alpha -Al2O3 occurred in the oxide scales and produced a significant decrease of the oxidation rate after an initial transient stage. This phase transformation was accelerated by microcrystallization.