Consideration is given to methods currently used for the determination of tensile and compressive fracture strains of thermally formed protective oxide layers at ambient and elevated temperatures. The methods discussed are four-point bend testing in both tension and compression and direct tensile and compression tests. Detection of failure of the oxide is usually by acoustic emission, but in some cases, where the acoustic signal is too small, direct observation of failure can be made byin situmethods using scanning electron microscopy or acoustic microscopy. Details of the tests are described together with the limitations of the different techniques. An attempt is made to estimate errors and consider the effects of important test variables such as strain rate, temperature and dwell periods.