Cluster ions composed of the alkyl nitriles, acetonitrile (CH3CN) and butyronitrile (C3H7CN), and the methyl cation (CH3+) have been examined in an effort to study methyl cation affinities and the intrinsic nucleophilicity of these bases. Structural characterization of the sas-phase dimeric adduct ions was achieved via multiple stage mass spectrometry (MSn) experiments and by quantum mechanical calculations. The kinetic method was used as a diagnostic tool in determining the structure of the dimeric adduct: the results of tandem mass spectrometry (MS2) experiments' are found to provide ratios which exclude loosely bonded dimers based on the thermochemistry of the constituent monomers, and which are consistent with a mixture of noninterconverting covalently bonded structures predicted by ab initio calculations. These clusters are bound such that one nitrile is N-methylated and the second nitrile is bound covalently to the carbon of the methylated cyano group. Collision-induced dissociation of this cluster ion results in the loss of a single neutral nitrile whereas both N-methylated nitriles should be formed upon dissociation of a loosely bound dimer with the greater fragment ion abundance corresponding to the nitrile having the higher CH3+ affinity. Ab initio calculations show a large barrier between the two isomeric forms of the dimeric cluster and this precludes intramolecular methyl cation transfer between the nitriles. The effects of fluorine substitution at the methyl cation, i.e., CH2F+ and CF3+, on the adducts of the nitriles greatly affects the stability order of the methylated nitrile monomers and dimeric adducts, and thus the abundance ratios of the MS/MS fragments. As the number of fluorine atoms in the cation is increased, the methylated nitrile becomes less stable relative to the dimeric cluster ion.104481129011296