We have synthesized graphite-saturated CHO fluids at 1273K and 2.4 GPa over 7.5 orders of magnitude in ƒ O 2. Bulk H/O of the fluid ranges from 1.78 (oxidized fluids in the H 2OCO 2 stability field) to 1330 (highly reduced fluids at the SiSiC buffer). The maximum H 2O content at 1273K and 2.4 GPa at the water maximum ( H/O = 2 ) is 95 ± 1 mol%. Molar H 2 is <0.05% in the CO 2 stability field ( H/O < 2 ), 1.5 ± 0.5% at the H 2O maximum, and 6.5 ± 1% in pure CH fluids. Molar C 2H 6 ranges from <0.05% at the H 2O maximum to 2% in a pure CH fluid. Molar H2/C2H6 and H2/CH4 are independent of the bulk H/O of the fluid over a wide range in H/O, indicating that internal reaction rates amongst fluid species are faster than H 2 loss. Experiments quenched with variable cooling rates show that CHO fluids preserve their high pressure-temperature species distribution during quenching, provided maximum quench rates are employed.