AbstractThe system H2— H2O has been studied isochorically from 0.5 to 90 mol‐% H2and up to 440°C and 2500 bar pressure using an autoclave containing two sapphire windows through which phase transitions could be observed at elevated temperatures and pressures. — The system was found to exhibit so‐called “gas‐gas” immiscibility with a critical curve proceeding to higher temperatures and pressures from the critical point of pure water. Within the range of these experiments, the critical temperature of H2—H2O mixtures does not change greatly from that of pure water (e.g.Tc= 381.3°C atpc= 2520 bar for 38 mol‐% H2).pVTmeasurements have been made in the homogeneous region and excess volumes have been calculated at 400°C and at different pressures. At 300 bar, the excess volumes are relatively large and positive (e.g.VE= 58.0 cm3mol−1at 40 mol‐% H2) whereas at 2500 bar, excess volumes of H2—H2O mixtures indicate only small positive or negativ