In-situ Auger electron spectroscopy was used to determine the 1:1 flux ratios of Ga and N during growth of GaN by molecular beam epitaxy at low substrate temperatures. By linearly ramping the Ga-flux, while keeping the N-flux constant, and simultaneously measuring the chemical composition by monitoring N and Ga Auger peaks, the time of deviation from stoichiometry could be determined. The method was applied at very low substrate temperatures where reflection high-energy electron diffraction does not reveal clear growth mode changes. The importance of the N- vs Ga-rich conditions were confirmed with transmission electron microscopy which showed a distinct change in crystallinity between material at the top and bottom of the film, which are in agreement with previous findings. Published by Elsevier B.V.