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In different production steps steels are exposed for short times, from a few minutes to up to a few hours, to high temperatures, either under reducing conditions for iron (e.g. N2-H2 during recrystallization annealing) or in oxidizing environments (e.g. during hot rolling and cooling after hot rolling). These processes are accompanied by external and internal oxidation, which can affect further processing steps. For example, during cooling after hot rolling of high strength steels especially grain boundary oxidation can lead to significant problems during pickling and cold rolling. For some alloys, e.g in the Fe-Mn-Si system, a surprisingly pronounced and deep grain boundary oxidation is observed. In this paper a fundamental investigation of grain boundary oxidation during annealing under model atmospheres simulating the oxygen partial pressure beneath the scale and also under significant lower partial pressure will be addressed, based on model alloys which are relevant to industrial high strength steel grades. The focus will be on questions such as how the different alloying elements and different annealing conditions influence the internal selective oxidation. In order to develop a fundamental understanding, the internal oxidation of dedicated binary and ternary model alloys (Fe-Mn, Fe-Si, Fe-Mn-Si, of different composition) was investigated in depth. Besides a thorough analysis of the oxide morphology, one important focus was on monitoring the mass gain in-situ during isothermal oxidation for short term exposures of up to 2h. The partial pressure of oxygen during the experiments was kept just below the equilibrium pressure for FeO/Fe and for comparison also at much lower partial pressure. As the mass changes during internal oxidation are very low, errors from buoyancy effects in a standard set-up are in the same range as the mass change to be investigated. Hence, a novel under-pressure TG set-up was developed and applied here. Furthermore, in-operando photoelectron spectroscopy of the oxide evolution at the surface by Ambient Pressure Photoelectron Spectroscopy (APXPS) was carried out, which provided important additional information for achieving a fundamental understanding of the observed oxidation kinetics. Based on the results the enhancing effect of the combination of Si and Mn on internal oxidation will be discussed.