CO2-bearing fluid inclusions coexisting with diamonds were identified in zircons from diamondiferous gneiss in the Kokchetav Massif. This discovery provides evidence for the presence of CO2 in UHP fluids and diamond formation in moderately oxidized conditions in the Kokchetav gneiss. Fluid and multiphase solid inclusions coexisting in zircons represent immiscible melt and fluid captured close to the peak metamorphic conditions for the Kokchetav UHP gneiss. Most of CO2-bearing inclusions are CO2+H2O mixtures except for some cases when they also contain daughter phases (e.g. muscovite, calcite and quartz) tracing the presence of aqueous and solute-rich fluids at different phases of UHP metamorphism. Decrease of pressure and temperature may have been responsible for the reduction of solutes in the CO2-bearing fluid. The lack of CO2-bearing inclusions in garnet porphyroblasts from diamond-bearing gneiss, as well as the common coexistence of aqueous CO2-bearing inclusions with calcite, testify that most likely all CO2 in fluid was consumed by the calcite-forming reaction and hydrous melt was the only remaining growth medium during retrograde metamorphism of the Kokchetav UHPM gneisses. Neither K-cymrite nor kokchetavite was identified among daughter phases in the hydrous melt inclusions in garnet, which indicates that they hardly could originate in a metapelitic system.