Protonated 3- and 4-nitropyrazoles were subject to collisional activation and neutralization-reionzation mass spetrometric studies using a large scale tandem mass spectrometer The gas phase basicities of 2-and 4-nitroimidazoles were determined by means of Fourier transform ion cyclotron resonance spectroscopy. These and other neutral and protonated molecules were studied by ab initio methods up to and including the CCSD(T)/6-31 + G* level. This information was used to assess the site of 3-nitro- and 4-nitropyrazole in the gas phase: at the equilibrium these compounds protonate on the heterocyclic nitrogen rather than on the oxygen of the nitro group.