The potential of carbonyl rhenium complexes in activating and coupling carbon dioxide and methane has been explored by using a combination of gas-phase experiments (FT-ICR mass spectrometry) and high-level quantum chemical calculations. While the complexes [Re(CO)(x)](++) (x = 0, 1, 3) are thermally unreactive toward CO2, [Re(CO)(2)](+) abstracts one oxygen atom from this substrate spontaneously at ambient conditions. Based on C-13 and O-18 labeling experiments, the newly generated CO ligand is preferentially eliminated, and two mechanistic scenarios are considered to account for this unexpected finding. The oxo complex [ORe(CO)(2)](+) reacts further with CH4 to produce the dihydridomethylene complex [ORe(CO)(CH2)(H)(2)](++). However, coupling of the CO and CH2 ligands to form CH2=C=O does not take place. Further, the complexes [Re(CO)(x)(CH2)](+) (x = 1, 2), generated in the thermal reaction of [Re(CO)(x)](+) (x = 1, 2) with CH4, are inert toward CO2. Mechanistic insight on the origin of this remarkable reactivity pattern has been derived from detailed quantum chemical calculations.