We report on studies of the As/GaN(0001) interface formation and its behaviour under the influence of annealing. The growth of As films and the impact of heating on the subsurface layer morphology were characterized in situ under ultrahigh vacuum conditions by X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (AHEM). The analysis gives a clear picture of the early stage of the formation of an As/GaN phase boundary showing that there is no Ga/As intermixing at the interface at room temperature, and As films have semiconductor features. Chemical interaction between the adsorbate and the substrate at elevated temperature was detected. A very small amount of As which remains on the surface followed annealing at 400 degrees C - 500 degrees C leads to the occurrence of surface states. Chemically reacted As atoms with the substrate surface are difficult to desorb even during annealing at 750 degrees C.