Ion chemistry at atmospheric pressure is of major relevance to novel methods for the abatement of volatile organic compounds (VOCs) that employ non-thermal plasmas. For this reason, positive and negative APCI (atmospheric pressure chemical ionization) mass spectra of all six di-, tri- and tetrachloroethanes diluted in air (500–1500 ppm) at atmospheric pressure were investigated at 30 °C and at 300 °C. Spectral changes due to collisional activation of the ions achieved by increasing ΔV, the potential difference between sampling and skimmer cones, are informative of structures and ion-molecule reactions. Positive ion chemistry of the chloroethanes (M) can, in general, be ascribed to C-C and C-Cl cleavages of the molecular ion, M+·, never detected but likely formed via exothermic charge exchange from primary ions of the APCI plasma. Exceptions to this characteristic pattern were observed for 1,1-dichloroethane and 1,1,2,2-tetrachloroethane, which give [M − H]+ and [M − HCl]+· species, respectively. It is suggested that both such species are due to ionization via hydride transfer. Upon increasing ΔV, the [M − HCl]+· ion formed from 1,1,2,2-tetrachloroethane undergoes the same fragmentation and ion-molecule reactions previously reported for trichloroethene. A nucleophilic reaction of water within the [C2H4Cl+](H2O)n ionic complexes to displace HCl is postulated to account for the [C2H5O+](H2O)m species observed in the positive APCI spectra of the dichloroethanes. Negative ion spectra are, for all investigated chloroethanes, dominated by Cl− and its ion-neutral complexes with one, two and, in some cases, three molecules of the neutral precursor and/or water. Another common feature is the formation of species (X−)(M)n where X− is a background ion of the APCI plasma, namely O2−,O3− and, in some cases, (NO)2−. Peculiar to 1,1,1-trichloroethane are species attributed to Cl− complexes with phosgene, (Cl−)(Cl2C=O)n(n = 1,2). Such complexes, which were not observed for either the isomeric 1,1,2-trichloroethane or for the tetrachloroethanes, are of interest as oxidation intermediates in the corona-induced decomposition process. No conclusions can be drawn in the case of the dichloroethanes, since, for these compounds, the ions (Cl−)(Cl2C=O)n and (Cl−)(M)n happen to be isobaric. Copyright © 2001 John Wiley & Sons, Ltd.