An adsorption study of Xe on both the bare Ge(100) surface, as well as on thin NaCl films grown pseudomorphically on Ge(100), was carried out using high-resolution low-energy electron diffraction. Ordered phases and phase diagrams were investigated in quasiequilibrium with room-temperature gas. Starting from low pressures, a transition from a disordered lattice gas to a distorted hexagonal phase was found for Xe on NaCl(100) in addition to the hexagonal bulk phase, which grows as a multilayer phase. The distorted hexagonal phase was found to be commensurate with the substrate in only one direction. At lower temperatures and/or higher pressures, the onset of three-dimensional growth is observed in the form of a hexagonal nanocrystalline phase (average grain size 80 Å) with Xe bulk lattice constant which is still oriented along the [110] direction with one of its axes. Both phase transitions are of first order, with pronounced hysteresis effects for the second. On Ge(100), no additional superstructure spots are induced by Xe before the bulk phase grows. The (2×1) reconstruction is maintained, but characteristic jumps in intensity of integral order as well as of half-order beams are observed as a function of temperature and pressure so that Xe seems to lock on defined sites in (2×1) registry. Phase diagrams for Xe adsorbed on both surfaces were derived and heats of transition at the phase boundaries were determined.