Prior to epitaxial deposition of gallium arsenide by molecular beam epitaxy it is necessary to remove surface oxide films in situ. Changes in the system pressure can be measured during the desorption process. The largest pressure changes are found for ion gauges that can directly measure the flux of arsenic atoms that are reflected or scattered from the substrate surface. It is found that there is a reproducible drop in apparent pressure of ∼25% as the oxide is desorbed, which is independent of whether an As2 or As4 overpressure is used to stabilize the surface. There are, however, qualitative differences between the two arsenic species, which can be attributed to the way they interact with the GaAs surface. Nevertheless, the pressure changes correlate precisely with the oxide removal, and they provide a reproducible monitoring tool for the calibration of substrate temperatures and the establishment of appropriate epitaxial growth conditions.