Special high-temperature and -pressure multinuclear NMR equipments were constructed, and used for the measurements of O-17-NMR chemical shift and spin-lattice relaxation time (T-1) in water over the range from liquid to supercritical (SC) states. The chemical shift could be interpreted in terms of the extent of hydrogen bonding. Although the cleavage of hydrogen bonding of water proceeds continuously from liquid to SC conditions, the hydrogen bonding was found to still remain even under SC conditions. It was confirmed that the spin-lattice relaxation of O-17 is mainly controlled by the quadrupole interaction and the T-1 values of O-17 are related with the values of the molecular reorientational correlation time (tau(c)) over the range from liquid to SC states. It was found that the tau(c) values decrease drastically with increasing temperature and decreasing density in lower temperature and higher density regions (25 T < 250 &DEG;C and 1.1 &GE; ρ > 0.6 g/cm(3)) and are close to the tau(c) corresponding to a free-rotor in sub-critical and SC ones (250 less than or equal to T less than or equal to 425 degreesC and 0.6 greater than or equal to rho greater than or equal to 0.1 g/cm(3)). It was considered from this result that a decrease of viscosity with increasing temperature and decreasing density leads to acceleration of molecular rotation in lower temperature and higher density regions, and on the other hand the rotational motions of water in sub-critical and SC regions are comparable to those of monometric water molecule. Accordingly, it was concluded that the principal contributions to the molecular rotational motions of water are the variation of the intermolecular hydrogen bonding interactions in lower temperature and higher density regions, and are that of the dipole moment of a water molecule in sub-critical and SC regions, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.