Highly reactive N-polar [000-1] GaN surfaces were analyzed using spectroscopic ellipsometry. Following exposure to air, observed changes in the pseudodielectric function near the GaN band edge indicate that surface contamination reduces the band bending. A subsequent Ga adsorption/desorption experiment on pristine N-polar GaN indicates that it contains a mixture of Ga-terminated and N-terminated surfaces. During deposition, Ga adatoms preferentially bond to the dangling bonds on the N-terminated surface: the measured 3.19 eV desorption activation energy equals the Ga-N decomposition energy. Further deposition forms a 1 ML Ga wetting layer whose 2.78 eV desorption activation energy is comparable to the Ga sublimation energy. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3054345]