In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ionmolecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (U VI O 2 2+ ) coordinated by either formate or acetate ligands E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom. , 2016, 27 , 19891998. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely UO 2 (O 2 CCH 3 ) 2 (O 2 CH) and UO 2 (O 2 CCH 3 )(O 2 CH) 2 . Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of UO 2 (H)(O 2 CCH 3 )(O 2 CH) . We also discovered that MS n CID of the acetateformate complexes leads to generation of the oxyl-methide species, UO 2 (O)(CH 3 ) , which reacts with H 2 O to generate UO 2 (O)(OH) . DFT calculations support the observation that formation of UO 2 (O)(OH) by elimination of CH 4 is favored over H 2 O addition and rearrangement to create UO 2 (OH) 2 (CH 3 ) . Experiment and theory show that loss of acetaldehyde is favored over formaldehyde during intramolecular hydride attack in uranyl complexes with accetate and formate ligands.