a See AcknowledgementsGlutathione (γ-L-glutamyl-L-cysteinyl-glycine) is a ubiquitous tripeptide found in all plants and animals. While a major participant in many biological reactions, glutathione has key roles as a metallochaperone, in the reduction of oxidative stress, in the transport of metabolites, and as a cellular thiol source. Here, we report its reaction with , dirhodium(II) tetraacetate (Rh2(OAc)4), a compound with anti-tumour properties, using electrospray ionization mass spectrometry and spectroscopic methods to observe in depth the stoichiometries of the final conjugate. Using computational analysis, we provide the first report of the orbital assignments for the resulting glutatathione-bound product. We also explore the competition by GSH for methionine-bound sites on Rh2(OAc)4 as a method for possible protection of its cellular-based therapeutic activity.