The f o 2 of the equilibrium between graphite and C-O fluid has been determined from 15-30 kbar and 1100-1400°C using a sliding redox sensor consisting of (Ni, Mn) O+Ni metal. The equilibrium composition of oxide coexisting with metal was approached from both directions in each experiment with convergence to within 1 mol% NiO. Since, in the P-T range of the experiments, C-O fluids are >90% CO2 our measurements of f o 2 translate into determinations of CO2 fugacity with an uncertainty of ±0.1 log units. These new determinations of the P-T-f o 2 plane of GCO equilibrium are in excellent agreement with the mainly unreversed measurements of Ulmer and Luth (1991) using pure metal-metal oxide sensors and with the equation of state of Saxena and Fei (1987). Modified forms of the Redlich-Kwong (MRK) equation of state (Holloway 1977; Flowers 1979; Kerrick and Jacobs 1981) predict higher values of f o 2 for the GCO equilibrium than determined experimentally. This implies that CO2 is more compressible than the MRK predicts.