The sodium adduct of disodium salts of benzene dicarboxylic acids (m/z 233), when subjected to collision-induced dissociation (CID), undergoes a facile loss of CO2 to produce an ion of m/z 189, which retains all the three sodium atoms of the precursor. The CID spectrum of this unusual m/z 189 ion shows significant peaks at m/z 167, 63 and 85. The enigmatic m/z 167 ion, which appeared to represent a loss of a 22-Da neutral fragment from the precursor ion is in fact a fragment produced by the interaction of the m/z 189 ion with traces of water present in the collision gas. The change of the m/z 167 peak to 168, when D2O vapor was introduced to the collision gas of a Q-ToF instrument, proved that such an intervention of water could occur even in collision cells of tandem-in-space mass spectrometers. The m/z 189 ion has such high affinity for water; it forms an ion/molecule complex even during the brief residence time of ions in collision cells of triple quadrupole instruments. The complex formed in this way then eliminates elements of NaOH to produce the ion observed at m/z 167. In an ion trap, the relative intensity of the m/z 167 peak increases with longer activation time even at the lowest possible collision energy setting. Similarly, the m/z 145 ion (which represents the sodium adduct of phenelenedisodium, formed by two consecutive losses of CO2 from the m/z 233 ion of meta- and para-isomers) interacts with water to produce a fragment ion at m/z 123 for the sodium adduct of phenylsodium. Other uncommon ions that originate also from water/ion interactions are observed at m/z 85 and 63 for [Na3O]+ and [Na2OH]+, respectively. Tandem mass spectrometric experiments conducted with appropriately deuterium-labeled compounds confirmed that the proton required for the formation of the [Na2OH]+ ion originates from traces of water present in the collision gas and not from the ring protons of the aromatic moiety. Copyright © 2010 John Wiley & Sons, Ltd.