Reactions occurring within the high-pressure mass spectrometer source during argon-enhanced negative-ion mass spectrometry (NIMS) of mono- and dichlorophenols result in the formation of adduct ions. The reactants for the formation of the adduct ions are derived solely from the chlorophenol. High-resolution accurate mass measurements of the adducts and comparison of the argon NIMS of the phenols with the argon-enhanced NIMS of authentic chlorinated phenoxyphenol and dichlorodioxin suggest that gas-phase intermolecular and intramolecular nucleophilic substitutions are occurring. The products of the source reactions reflect the stability of the gas-phase species involved and may be compared with the photolysis and pyrolysis reaction products of chlorophenols, chlorinated phenoxyphenols and chlorophenates reported in the literature.