Microwave spectroscopy that can be applied to study the dielectric relaxation of various fluids under high temperature and pressure has been developed in the frequency range up to 40 GHz. By utilizing this new technique, the dielectric relaxation of water has been measured in the temperature and pressure range up to 750 degrees C and 120 MPa, which corresponds to a density range between 0.05 and 1 g/cm(3). The static dielectric constant epsilon(0) is deduced from the time required for a microwave signal to travel through the sample by means of the time domain analysis, and is in good agreement with the literature. The dielectric relaxation time tau is obtained by fitting the experimentally observed microwave transmission rate to the value calculated using the S-matrices on the assumption that the dielectric constant obeys the Debye relaxation. The results of tau at ambient pressure agree very well with previous data. Below about 350 degrees C, tau rapidly decreases with increasing temperature nearly independent of pressure, while above about 350 degrees C, tau changes little with temperature and increases rapidly with decreasing density. It is concluded that the most relevant parameter determining tau is the temperature at lower temperatures or higher densities, and it is the density d at higher temperatures or lower densities. A possible change in the nature of hydrogen bonding is suggested to explain the observed temperature and density dependence of tau. (C) 1997 American Institute of Physics.