The long suspected presence of two isomeric copper complexes (A and B), proportionally dependent on ESI cone voltage, was confirmed by their isolation, separation and characterization in a travelling wave ion mobility spectrometry-mass spectrometer (TWIMS-MS) with modifications to allow for ion/molecule reactions to be performed. Despite their small difference in cross-sectional area (similar to 1%) the isomers were well resolved, in part due to a strong interaction of isomer B with the N-2 buffer gas. The reactivity of each TWIMS-separated isomer was probed: propane reacted with isomer A via C-H bond activation and O-atom transfer, while no such reactions were observed for isomer B; the aryl halides PhX (X = F, Cl, Br, I) also reacted solely with A, via either concerted oxidation and halide transfer, or O-atom transfer. DFT calculations reveal that the aryl C-X bond activation of PhX by isomer A involves several mechanistic pathways. (C) 2014 Elsevier B.V. All rights reserved.