Helium and light scattering experiments have been performed to study the formation of water-ammonia ice layers on graphite at 110-160 K. Ice formation is mainly governed by the partial water pressure, but the presence of ammonia enhances the formation rate and influences the layer properties. Ice formed at temperatures <140 K wets the surface, while ice formed at higher temperatures does not. At temperatures around 140 K ammonia destabilizes the two-dimensional ice layers and a dewetting transition is observed at a surface coverage of 90-95%. (C) 2007 Elsevier B.V. All rights reserved.