Adsorption process of ammonia on the Zn-terminated ZnO(0001) surface at room temperature has been studied by photoelectron spectroscopy utilizing synchrotron radiation. Coverage-dependent measurements of the N 1s core-level peak and the work function change have been carried out. It is revealed that ammonia adsorbs molecularly in the initial stages of adsorption, whereas deprotonated species are formed after some amount of molecular ammonia is accumulated on the surface. The reactivity of the K-modified ZnO(0001) surface towards ammonia adsorption has also been investigated and found to be significantly lowered by predeposited K. (C) 2004 Elsevier B.V. All rights reserved.