Symmetry induced kinetic isotope effects (SIKIE) have been observed mass spectrometrically in the termolecular association reaction of Ar and CO?2?+ to produce Ar . CO2+ making this the first example of SIKIE in a system where the reactants have no atoms in common. The O-18 SIKIE shows a pronounced dependence on whether the CO2+ are produced directly by electron ionization or indirectly by charge transfer from Ar+. The results can be understood generally in terms of reaction restrictions based on a symmetry correlation scheme recently developed by one of the authors. This scheme indicates that CO2+ ions can have very different clustering reactivity with Ar depending on whether the ion is in an e (restricted) or an f (allowed) parity label state. Kinetic modeling of the O-18 SIKIE indicates that CO2+(e)/CO2+(f)approximate to 60 for CO2+ produced by 21 eV electron ionization.