Because of the high stability and inertness of the U=O bonds, activation and/or functionalization of UO 2 2+ and UO 2 + remain challenging tasks. We show here that collision-induced dissociation (CID) of the uranyl-propiolate cation, [U VI O 2 (O 2 C-C≡CH)] + , can be used to prepare [U VI O 2 (C≡CH)] + in the gas phase by decarboxylation. Remarkably, CID of [U VI O 2 (C≡CH)] + caused elimination of CO to create [OU VI CH] + , thus providing a new example of a well-defined substitution of an "yl" oxo ligand of U VI O 2 2+ in a unimolecular reaction. Relative energies for candidate structures based on density functional theory calculations suggest that the [OU VI CH] + ion is a uranium-methylidyne product, with a U≡C triple bond composed of one σ-bond with contributions from the U df and C sp hybrid orbitals, and two π-bonds with contributions from the U df and C p orbitals. Upon isolation, without imposed collisional activation, [OU VI CH] + appears to react spontaneously with O 2 to produce [U V O 2 ] + . Graphical Abstract .