The gas-phase reactions of ClMn(H2O)(+) with a variety of volatile and nonvolatile, saturated and unsaturated hydrocarbons have been examined by using Fourier transform ion cyclotron resonance mass spectrometry (FT/ICR). The ClMn(H2O)(+) ion reacts rapidly by exclusive H2O ligand displacement with all the hydrocarbons studied, including highly branched alkanes that usually fragment upon ionization. These observations are rationalized on the basis of the electronic structure of ClMn+. Collision-activated dissociation of the product ions provides structural information which promises to allow the distinction and structural characterization of isomeric hydrocarbons. These findings suggest that the ClMn(H2O)(+) ion is a highly promising chemical ionization reagent for mass spectrometric characterization of hydrocarbons, including those that commonly exist in petroleum