Methyl iodide (CH 3 I) has been used as a chemical ionization reagent gas and the ion/molecule reactions of formed reagents ions with a number of nucleophiles were studied in a chemical ionization source of a conventional mass spectrometer. Generalizing, charge exchange reactions dominate for compounds with lower ionization energy than CH 3 I, while the others preferred protonation and methylation reactions. For ketones both protonation and charge exchange reactions were thermodynamically possible and the prevailing reaction was dependent on the compound. Surprisingly, all the isomeric C 5 ketones gave rise to clearly different spectra. Alcohols, having higher ionization energies than CH 3 I and a lower proton affinity than that of CH 2 I• , also reacted interestingly. Ionization through hydroxide abstraction prevailed for C 3 -C 6 alkanols. Chemical ionization (CI) mass spectrometry is an adaptable method for examining ion/molecule reactions in the gas phase. In addition, molecular weight and structural information can be obtained by a suitable choice of reagent gas. The gas-phase reactions between alkyl halides and various nucleophiles have aroused interest for a long time. The reasons for this may be that in the condensed phase alkyl halides are known to be good substrates for nucleophilic substitution, one of the most useful class of organic reactions. The main topics in the area of gas-phase ion/ molecule chemistry of methyl and ethyl halides are the unimolecular chemistry of the pure alkyl halide, [1][2][3][4] reactions of alkyl halides with various nucleophiles [5][6][7][8] and especially the site of cation attachment in the course of electrophilic addition reactions.9-18 The regioand stereoselective reactions of alkyl halides towards geometric isomers have also been probed. [19][20][21][22] As a curiosity, the formation of [M + 13] + ion especially for aromatic compounds has attracted interest. [23][24][25] Highpressure radiolytic techniques have also been applied to study the chemistry of dimethylhalogenium ions in the gas phase.