In the course of this work, the influence of atmospheres containing water vapor on high-temperature oxidation of pure iron and Fe-Si alloys with up to 3.2% silicon in the temperature range between 900 °C and 1200 °C was investigated. According to the findings in the literature, water vapor influences the diffusion processes and the plastic behavior of the oxide layer formed. In order to perform targeted experimental studies of high-temperature oxidation under laboratory conditions, three new pieces of laboratory equipment were put into operation: a water vapor generator for gas humidification, a suitable furnace for simultaneous thermal analysis, and a mass spectrometer for gas analysis. More than 40 thermogravimetric (TG) tests were performed between 900 and 1200 °C under dry synthetic air and under N2-O2-H2O or N2-H2O gas mixtures containing water vapor. Metallographic investigations showed that a lift-off of the oxide layer occurring under dry atmospheres is strongly reduced in the test under water vapor atmospheres with increasing H2O content. Due to the better contact at the metal-oxide interface and the higher mass transfer, this results in an increased total oxidation of pure iron. On the high-temperature oxidation of Fe-3.2%Si alloys, water vapor loading of synthetic air showed no significant effect. From the results of the TG investigations, the relevant oxidation parameters were determined and summarized in a kinetic law. This offers the possibility to calculate the mass changes and oxide layer thicknesses of comparable oxidation tests. These formulas work reliably for pure iron under dry and water vapor loaded synthetic air. Estimates for iron-silicon alloys are possible, but further experimental results are needed to increase accuracy.