We report Pt-II-to-Au-I methyl transfer reactions that occur in the gas phase and in solution. The heterobimetallic Pt-II/Au-I complexes {[(dmpe)PtMe2][AuPR3]}(+) (R = Me (2a), Ph (2b), Bu-t (2c)), observed in the gas phase by means of electrospray ionization, were subjected to collision induced dissociation (CID) from which we could observe Pt-to-Au transmetalation along two reaction pathways involving formation of a Au-Me bond, analogous to those observed for the Pt-II/Cu-I complex recently reported. In the first pathway, neutral AuMe is generated with concomitant migration of PR3 from Au-I to the Pt-II center, forming cation [(dmpe)PtMe(PR3)](+) (R = Me (5a) or Ph (5b)). In the second pathway, the monophosphine stays attached to the gold center, yielding cation [(dmpe)PtMe](+) (7) and R3PAuMe. Quantitative energy-resolved collision induced dissociation experiments as well as density functional theory (DFT) calculations were used to investigate the potential surface involved in the transmetalation processes. Energy barriers of 22.3 and 47.9 kcal mol(-1) for the two reaction processes of 2b and of 45.4 kcal mol(-1) for the single reaction process of 2c were obtained. Parallel reactivity is observed in THF solution, allowing for a comparison of the product distributions with those observed in the gas phase, and the postulation of simple steric control of the branching ratio between the two pathways. DFT calculations at the M06-2X//BP86/TZP level were in good agreement with the experiments.