In this paper, the factors influencing the oxidation resistance of superalloys are studied. A model is proposed by which the Al2O3-forming properties of a given composition can be estimated, based upon the thermodynamic and kinetic factors influencing scale growth. The numerical modelling is tested by experimental work on a number of compositional variants of the newly-developed SCA425+ superalloy, which contains appreciable quantities of Cr. The modelling is shown to be in broad agreement with experiment. The effects of Al, Cr and Si on the oxidation resistance of this class of alloy have been rationalised.