Surface contaminations on GaAs (100) wafers have been analyzed by means of temperature‐dependent and angle resolved x‐ray photoelectron spectroscopy (TOXPS, ARXPS), and time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). First, the dependence of the composition, chemical state, and diffusion behavior of the passivating thermal oxide on different annealing temperatures was investigated. The phases of this oxide can be described by a multilayer model and are consistent with the Ga–As–O equilibrium phase diagram. Second, the nature and amount of impurities at ambient and elevated temperatures was studied. No metal contaminants were found within the sensitivity of TOF‐SIMS. The residual amount of carbon contamination at the desorption temperature of the oxide depends on the amount of photon irradiation at ambient temperature.