A study has been performed into the high temperature oxidation of an AISI 304 stainless steel in order to explain the so-called "breakaway" oxidation behaviour observed on chromia-forming alloys. The high temperature in situ X-ray diffraction technique was used to detect the structural changes during the high temperature oxidation process at 900 and 1000degreesC. Chromia-forming alloys generally develop dense and protective slow growing Cr2O3 layers. This low growth rate is effectively observed at 1000degreesC and is favoured by the establishment of a continuous silica sub-layer at the internal interface. This acts as a barrier against iron diffusion. "Breakaway" oxidation behaviour is observed at 900degreesC after 40 hours of oxidation. This phenomenon is related to the initial nucleation of Fe7SiO10, which traps silicon within the oxide layer and inhibits its segregation at the internal interface. The high rate oxidation is then due to the lack of a continuous silica layer at the internal interface, which leads to the formation of less protective iron-containing oxides.