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Notes: Abstract Natural, pure CO2 inclusions in quartz and olivine (c. Fo90) were exposed to controlled fH2 conditions at T= 718–728°C and Ptotal= 2 kbar; their compositions were monitored (before and after exposures) by microsampling Raman spectroscopy (MRS) and microthermometry. In both minerals exposed at the graphite–methane buffer (fH2= 73 bar), fluid speciations record the diffusion of hydrogen into the inclusions. In quartz, room-temperature products in euhedral isolated (EI type) inclusions are carbonic phases with molar compositions of c. CO2(60) + CH4(40) plus graphite (Gr) and H2O, whereas anhedral inclusions along secondary fractures (AS type) are Gr-free and contain H2O plus carbonic phases with compositions in the range c. CO2(60) + CH4(40) to CO2(10) + CH4(90). EI type inclusions in olivine evolved to c. CO2(90–95) + CH4(5–10) without Gr, whereas AS type inclusions have a range of compositions from CO2(90) + CH4(10) ± Gr to CH4(50) + H2(50) ± Gr; neither H2O nor any hydrous species was detected by optical microscopy or MRS in the olivine-hosted products. Differences in composition between and among the texturally distinct populations of inclusions in both minerals probably arise from variations in initial fluid densities, as all inclusions apparently equilibrated with the ambient fH2. These relations suggest that compositional variability among inclusions in a given natural sample does not require the entrapment of multiple generations of fluids. In addition, the absence of H2O in the olivine-hosted inclusions would require the extraction of oxygen from the fluids, in which case re-equilibration mechanisms may be dependent on the composition and structure of the host mineral.Many of the same samples were re-exposed to identical P–T conditions using Ar as the pressure medium, yielding ambient fH2= 0.06 bar. In most inclusions, the carbonic fluids returned to pure CO2 and graphite persisted in the products. Reversal of the mechanisms from the prior exposure at fH2= 73 bar did not occur in any inclusions but the AS types in olivine, in which minor CO2 was produced at the expense of CH4 and/or graphite. The observed non-reversibility of previous mechanisms may be attributed to: (1) slower fluid–solid reactions compared to reactions in the homogeneous fluid phase; (2) depressed activities of graphite due to poor ordering; and/or (3) low ambient fO2 at the conditions of the second run.