The decomposition of ammonia adsorbed on the 211, 100 and 110 tungsten planes was studied by means of a probe-hole field emission microscope, at low pressures and over a wide range of temperatures. The experimental system consisted of a field emission tube combined with a molecular beam apparatus. The results obtained indicate that when ammonia interacts with a clean surface decomposition begins below 300 K and is completed above 400 K on all planes, the only products being hydrogen and nitrogen. On the 211 and 111 planes both nitrogen are initially adsorbed. On the 100 plane only nitrogen remains above 400 K. At about 600 K nitrogen leaves the 211 plane by surface diffusion. This causes an increase of nitrogen coverage on the 111 plane. When NH₃ is adsorbed onto the 100 plane, precovered with nitrogen, the decomposition process terminates at higher temperatures compared with the clean surface. Species containing hydrogen are formed and are most probably bound to the surface via N—N bonds. These species decompose above 600 K to give nitrogen in the β₁ state in addition to the existing β₂ layer. On the 110 plane incomplete results were obtained owing to difficulties caused by field desorption.