The presence of CO2 in the upper mantle has been inferred from several sources of evidence. To aid the understanding of the behavior of CO2 at high pressures and temperatures, CO2 fugacities have been deduced from the following decarbonization reactions: MgCO3+SiO2=MgSiO3+CO2, MgCO3+MgSiO3=Mg2SiO4+CO2, MgCO3+TiO2=MgTiO3+CO2, and CaCO3+SiO2=CaSiO3+CO2. The deduced fugacities indicate that CO2 is more compressible at high pressures than previously predicted. This discrepancy could cause serious errors in the calculation of stabilities of mineral assemblages under upper mantle conditions.