Abstract In graphite-buffered COH systems P , T and CO 2 CH 4 ratio fix the partial pressures of all the gas species. The relation between CO 2 CH 4 ratio and H 2 O content of the fluid was calculated for temperatures of 300°, 400°, 500°, 600°, 700° and 800°C and pressures of 500, 1000, 2000, 5000 and 9000 bar. It appears that water-deficient fluid compositions can only occur at very high or at very low CO 2 CH 4 ratios. Many of the data published on CO 2 inclusions in medium- and high-grade metamorphic terranes are incompatible with the calculated diagrams. Possible explanations are discussed. The assumption that fluid compositions trapped in inclusions are generally preserved needs further investigation.