Stress in the early stages of growth has been measured in α-Al2O3 (alumina) scales formed on FeCrAl- and NiAl-based alloys during heating in air at 1000 °C to 1200 °C. Scale thickness ranges from 0.5 to 5 μm, times from 5 to 720 min. Stress was measured using the multiple-tilt method. In order to measure the thinnest scales at the earliest times, focused, monochromatic synchrotron radiation was used for high intensity, and a fixed, small angle of incidence was used along with an appropriate wavelength to maximize scattering from the film relative to the background from the substrate. Depending on the composition, transient tensile stresses of up to 1.2 GPa were observed, with maximum stress at times ranging from >10 h at 1000 °C to <10 min at 1200 °C. Thermal stresses induced by an abrupt temperature change were found to relax much more quickly, suggesting that the kinetics observed during isothermal growth reflect a dynamic competition between stress generation and stress relaxation. These results challenge commonly accepted models of growth stress in scales that predict that a compressive stress will be generated as the metal converts to a larger-volume oxide in a constrained location such as an interface. The observed tensile stress may be due to another mechanism altogether (e.g., grain coalescence), or to the conversion of a transitional Al2O3 to the equilibrium α-Al2O3 phase. For one composition, transitional Al2O3 is observed during the period of tensile stress