We report on in situ resistivity measurements on Ga1-xMnxAs during post-growth annealing in different atmospheres. A drop in the resistivity is observed when the Ga1-xMnxAs is exposed to oxygen, which indicates that the passivation of Mn interstitials (Mn-I) at the free surface occurs through oxidation. The presence of oxygen can therefore be an important annealing condition for the optimization of Ga1-xMnxAs thin films, all the more since the oxidation appears to be limited to the sample surface. Annealing in an oxygen-free atmosphere leads to an increase in the resistivity indicating a second annealing mechanism besides the outdiffusion of Mn-I. According to our magnetization and Hall effect data, this mechanism reduces the amount of magnetically and electrically active Mn atoms. (C) 2005 American Institute of Physics.