Production of ammonia by surface reactions of H and N atoms on surfaces not wetted by partially ionized plasma may represent an important technological issue in fusion reactors where puffing nitrogen is employed to cool plasma in the divertor region. The H and N atoms are likely to interact on such surfaces forming NH3 molecules. The interaction efficiency was studied in a laboratory setup consisting of two separate sources of either N or H atoms. Both sources enabled experiments with atoms at room temperature in the range of H-atom density of the order of 1021 m−3 and N-atom density of the order of 1020 m−3. The production of ammonia was measured with a calibrated residual gas analyser. The production depended on the fluxes of both atoms onto the surface of selected materials. As a general rule, the higher H-atom flux at a constant N-atom flux caused an increase in ammonia production. The highest efficiency of up to 50% was found for nickel. It was up to 30% for tungsten, whereas for P92 alloy, it was up to about 20%. The accuracy of these results is within about ±20% of the measured values. Methods for suppressing ammonia formation in fusion reactors will have to be invented in order to enable appropriate long-term operation.