In order to fundamentally understand cobalt catalyst deactivation in Fischer-Tropsch synthesis (FTS) due to parts per million levels of NH3 in the synthesis gas, the adsorption and decomposition of NH3 on Co(0001) are investigated experimentally under ultrahigh vacuum (UHV) conditions and theoretically using density functional theory (DFT) calculations. NH3 desorbs intact from the surface, between 100 and 270 K. In agreement with this, DFT calculations show that the activation barrier for NH3 decomposition, 105 kJ/mol, is higher than the adsorption energy of NH3, 59 kJ/mol. Neither COad nor Had block the adsorption of NH3. Instead, CO and NH3 form a stable coadsorbed layer. Preadsorbed ammonia negatively affects dissociative H2 adsorption. Electron-induced dissociation produces NHx species on the surface at low temperature. The order of stability is NH(+2 Had) > N(+3 Had) > NH2(+ Had) > NH3. N and NH lower the quantity of CO that can be accommodated on the surface but do not affect the adsorption energy significantly. For FTS, we conclude that (i) NH3 adsorption on cobalt is not inhibited by the other FTS reactants and thus parts per million levels of NH3 can already be detrimental, (ii) due to their high stability, NHx species are most likely responsible for catalyst deactivation.