Even though oxidation kinetics of low-alloy steels used in power plants at temperatures below 550degreesC have been evaluated in many studies, there are important details that are not fully understood yet. Consequently, a mechanism-based simulation of the oxidation process is not possible. In the framework of the EU project OPTICORR, the present study focuses on the thermogravimetric analysis of the oxidation kinetics of three low-alloy steels with Cr-concentrations between 0.55 and 2.25 wt% in laboratory air. By means of analytic scanning electron microscopy and X-ray diffraction growth kinetics and chemical composition of the outer (mainly Fe3O4) and inner layer (Fe3O4 + FeCr2O4) were separated. The influence of the substrate microstructure on the inward scale growth via grain-boundary diffusion was studied by preparing specimens with different grain size. To model the growth of the inner layer the diffusion processes were treated numerically by the implicit finite-difference method.