In this study, a new experimental approach for the observation of hydothermal reactions and phase transitions using a new design hydrothermal diamond anvil cell (HDAC) with in situ infrared (IR) spectroscopy at high temperatures and pressures has been developed. Experimental observations of the phase transition are carried out in the temperature range 25-850degreesC and over a pressure range from 1 atm to about 3-5 GPa. The HDAC was connected to both a light and an IR microscope. For a case study, halite-water system was the experimental example. For the NaCl-H2O system, a supercritical phase (F), liquid phase (L), vapor phase (V), solid phase (H), L + H, L + V and H + L + V phase fields were observed. The diamond cell has a wide-angle window, which allows for an IR beam to pass through. During heating from low temperature to high temperature, each phase and the phase transition process were examined through in situ light and FT-IR microscope. To avoid a strong absorbency of H2O, the H2O-D2O-NaCl system, in which there is a low percentage of H2O, has been also tested by in situ FT-IR spectroscopy. A strong shift of the vibration spectra of the aqueous solution close to the critical temperature of water has been observed. This observation system has many advantages in determining the volume, temperature and pressure of the sample chamber and doing quantitative analyses of the sample composition and molecular structure by examining FT-IR spectra. (C) 2003 Elsevier B.V. All rights reserved.