Using a volumetric technique, the hydrogen solubility X in liquid water (L), low-pressure hexagonal ice (I _h), and high-pressure cubic clathrate ice (sII) is studied at hydrogen pressures up to 1.8 kbar and temperatures from−36 to+20 ^°C. The triple point of the L+I _h+sII equilibrium is located at P=1.07(5) kbar and T=−10(1) ^°C. The hydrogen concentrations of phases at the triple point are X _L =0.17(5), and X _sII=2.3(1) wt.% H₂. The thermal stability and the process of decomposition of the clathrate phase at ambient pressure are studied by neutron diffraction.

The transition of the hexagonal ice phase I _h to the clathrate phase sII has been found in the H₂O-H₂ system at a pressure of about 1 kbar under conditions of an excess of gaseous hydrogen. The pressures of the I _h → sII and sII → I _h transitions have been determined over a temperature range from −36 to −18°C, and the pressure dependence of the synthesis temperature of the clathrate phase from a liquid at pressures from 1.0 to 1.8 kbar has been constructed. The solubility of hydrogen in the I _h and sII phases and in liquid water has been measured. The concentration of hydrogen in the clathrate phase sII is about 1.2 wt % (10 mol %) near the boundary of the sII → I _h transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar.

Using a volumetric technique, phase transitions in the H₂O–H₂ system were investigated in the vicinity of two points of an invariant equilibrium, L+I _h+sII and L+sII+C ₁, located at 1.07 kbar and−10 °C and at 3.6 kbar and 1 °C, respectively. Liquid water (L), low-pressure hexagonal ice (I _h) and high-pressure cubic (sII) and rhombohedral (C ₁) clathrate hydrates were in equilibrium with gaseous hydrogen taken in excess.