In order to reveal the nature of deep crust fluids, the phase relations of NaCI-saturated solution at high temperatures and pressures in a hydrothermal diamond anvil cell (HDAC)are investigated. Salinity of the solutions observed is about 35%—50%. The temperatures for the observation range from 25 to 850℃ and the pressures from 1 atm to about 1 GPa. A supercritical single phase, liquid phase (L), vapor phase (V), solid phase (H), L + H, H + V + L and the near-critical phases L + V can be observed. A two-phase (L + V) immiscibility field for the NaCI solution has been discovered to lie in a wide range of temperatures and pressures:from 250( ± 3) to 721℃. Within this field there are two parts, where the upper high temperature part of the two-phase regions is very unstable in character. It is possible to observe a “critical phenomenon”. In some of our experiments an “explosion” almost constantly occurred at the interface between the liquid and vapor phases, making the interface obscure, and a continuou In order to reveal the nature of deep crust fluids, the phase relations of NaCI-saturated solution at high temperatures and pressures in a hydrothermal diamond anvil cell (HDAC) are investigated. temperatures for the observation range from 25 to 850 ° C and pressures from 1 atm to about 1 GPa. A supercritical single phase, liquid phase (L), vapor phase (V), solid phase (H), L + H, A two-phase (L + V) immiscibility field for the NaCI solution has been discovered to lie in a wide range of temperatures and pressures: from 250 (± 3 Within this field there are two parts, where the upper high temperature part of the two-phase regions is very unstable in character. It is possible to observe a “critical phenomenon ”. In some of our experiments an “explosion ” almost constantly occurred at the interface between the liquid and vapor phases, making the interface obscure, and a continuou