Reactions of M+(H2O)n (M=V, Cr, Mn, Fe, Co, Ni, Cu, Zn; n≤40) with NO were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Uptake of NO was observed for M=Cr, Fe, Co, Ni, Zn. The number of NO molecules taken up depends on the metal ion. For iron and zinc, NO uptake is followed by elimination of HNO and formation of the hydrated metal hydroxide, with strong size dependence. For manganese, only small HMnOH+(H2O)n−1 species, which are formed under the influence of room-temperature black-body radiation, react with NO. Here NO uptake competes with HNO formation, both being primary reactions. The results illustrate that, in the presence of water, transition-metal ions are able to undergo quite particular and diverse reactions with NO. HNO is presumably formed through recombination of a proton and 3NO− for M=Fe, Zn, preferentially for n=15–20. For manganese, the hydride in HMnOH+(H2O)n−1 is involved in HNO formation, preferentially for n≤4. The strong size dependence of the HNO formation efficiency illustrates that each molecule counts in the reactions of small ionic water clusters.