TiAlSiN hard coatings are commonly known to exhibit a high oxidation resistance, however, the influence of a varying Al content on the oxidation mechanism has not yet been examined in detail. Thus, in this work, the temperature dependent phase composition of two powdered TiAlSiN coatings with low Al contents (Ti37Al2Si7N54 and Ti33Al6Si7N54) was evaluated by in-situ X-ray diffraction (XRD) in ambient air up to 1200 °C and subsequent Rietveld refinement complemented by differential scanning calorimetry measurements. The in-situ XRD experiments revealed the formation of metastable anatase TiO2 during oxidation for both TiAlSiN coatings, however, the maximum wt% of this phase was found to be doubled for the coating with higher Al content from 9 to 21 wt% at ~1025 °C. Furthermore, the microstructure of the compact Ti33Al6Si7N54 coating oxidized at 950 and 1100 °C was investigated comprehensively by means of XRD, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. These microstructural investigations revealed the formation of a thin protective Al2O3 layer, which was broken by the growth of TiO2 grains after oxidizing at 950 °C, leading to fully enclosed Al2O3 grains within coarsened TiO2 grains at 1100 °C. The present work allows to close the literature gap concerning the changes in oxidation mechanism of TiAlSiN coatings when exclusively varying the Al content and further illuminates the microstructure of an oxidized TiAlSiN with low Al content in detail.