The isothermal oxidation of Ni-base single-crystal superalloy AM1 was investigated for up to 3600 h at 850 °C and 900 °C. The aim of the study was to test an existing model of oxidation kinetics that considers transitory oxide growth. The samples were characterized at various intervals to correlate the microstructure of the oxide scale with the oxidation kinetics. Transition alumina ( θ ) was observed among other transition oxides such as spinel, rutile, and chromia, which helped in understanding the nature and kinetics of the transitory stage. After a sufficiently long duration, all samples formed a continuous α -alumina layer at the metal/oxide interface. The previously published model, based on three kinetic parameters, was validated in the temperature range of 800–1200 °C. The duration of the transient regime characterized in this study at 850 °C and 900 °C was consistent with the kinetics model, with a slight increase in the value of the model parameter describing the lateral growth kinetics of α -alumina. This modification resulted in a slight reduction in the duration of the transient regime at low temperatures.