The chemisorption of MH, on the GaN(0001)-(1 X 1) surface has been studied using ultraviolet photoemission spectroscopy together with semiempirical molecular orbital cluster calculations. Adsorption removes surface states near the valence band maximum with little or no change in band bending. The data indicate a much smaller emission intensity for the N lone-pair orbital than is expected for an -NH2 species with three-fold coordinated N. Comparison of observed and calculated spectra suggests that NH3 dissociates into chemisorbed NH2 and H, with the N atom four-fold coordinated as an asymmetric bridge between two surface Ga atoms.