The gas phase reactivities of small cationic iron clusters, Fe_n⁺ (n = 1–20), towards ammonia were investigated using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Sequential addition of ammonia molecules to the clusters was observed to be the dominating process for n > 4. In the case of n = 4 we observed addition of ammonia accompanied by dehydrogenation. This reaction was modelled using hybrid density functional theory. Clusters with n < 4 do not react with ammonia. Clusters Fe_n⁺ (n = 1–20) react with neither N₂ nor H₂ at around 10⁻⁸ mbar. When dinitrogen was seeded into the expanding helium, mixed clusters of the type Fe_nN_m⁺ were observed. These ions react with H₂, either by addition, or by substitution of N₂. The clusters with m = 1 were isolated in separate experiments and reacted with H₂, which showed that mixed clusters with n = 5–13 add up to 5 molecules of dihydrogen in successive slow reactions.