Abstract. Strains in thermally grown oxides have been measured in-situ, as the oxides develop and evolve. Extensive data have been acquired from oxides grown in air at elevated temperatures on different model alloys that form Al 2 O 3 . Using synchrotron x-rays at the Advanced Photon Source (Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from the oxidizing specimen were recorded every 5 minutes during oxidation and subsequent cooling. The diffraction patterns were analyzed to determine strains in the oxides, as well as phase changes and the degree of texture. To study a specimen's response to stress perturbation, the oxidizing temperature was quickly cooled from 1100 to 950 o C to impose a compressive thermal stress in the scale. This paper describes this new experimental approach and gives examples from oxidized β-NiAl, Fe-20Cr-10Al, Fe-28Al-5Cr and H 2 -annealed Fe-28Al-5Cr (all at. %) alloys to illustrate some current understanding of the development and relaxation of growth stresses in Al 2 O 3 .Introduction. The existence of a growth stress associated with oxidation has long been envisioned [1], since most oxides have higher volumes than their corresponding metals. Later, it was proposed that growth stress can arise from oxide formation within the scale as the diffusing cations and anions meet and react, particularly along oxide grain boundaries [2]. Several other mechanisms have also been proposed to explain the origin of growth stresses [3]; however, actual in-situ measurements of these stresses are scarce. Values are usually deduced from the difference between measured residual stresses at room temperature and the calculated thermal stress based on thermal expansion coefficients (CTE) of the oxide and the alloy. However, this method depends on CTE values, which are often unreliable, cannot properly account for stress relaxations and fails to follow the development of growth stresses with time.