This paper concerns microstructural characterization of alumina scales formed on a 2nd generation single crystal Ni superalloy during isothermal oxidation in dry oxygen at 1050, 1100 and 1150 °C for 100 h. Samples for high resolution characterization of Al₂O₃ scales were prepared using a focused ion beam (FIB) method. High resolution TEM and S/TEM techniques were used for a detailed characterization and a direct comparison of the phase composition and chemistry of the oxide scales formed during high temperature oxidation. The growth of transient θ-Al₂O₃ and its transformation to α-alumina is addressed for each oxidation temperature along with the differences in the diffusion of reactive elements, such as Hf, Zr and Y, through grain boundaries of the α phase. The θ to α transformation front was proven to move from the metal-scale to the scale-gas interface. The results presented in this paper indicate that after 100 h of oxidation at 1050 and 1100 °C there are still some θ-alumina grains remaining and even in the regions where the transformation to α was finished the surface retained the whisker-like morphology.