Vibrational Raman spectra of O-2 corresponding to the Q-branch of fundamental and hot bands were measured in supercritical water at 380-500 degreesC and 5.0-39.2 MPa where the corresponding density was ranged in 0.0009-0.031 mol cm(-3) so as to investigate the local water structure around O-2. The band shapes were asymmetric and looked like the gas phase spectrum at high density that consisted of rotational lines broadened by pressure. They were reasonably reproduced by the band shape calculation using the so called G-matrix, which considers the overlapping effect among the rotational lines. This implies that the rotational relaxation of O-2 in SCW is not largely different from the gas phase behavior. In the O-2 (10 mol%)-H2O Mixture at 380-500degreesC, the Raman vibration-rotation shifts decreased linearly dependent on the total density of mixture up to 0.014 mol cm(-3) (rho(r) < 0.7), indicating the absence of large clustering or depletion of water around O-2 under the experimental conditions. In the O-2 (1 mol%)-H2O mixture at 390degreesC (T-r = 1.02), the Raman shifts also decreased linearly to the total density up to ca. 0.01 mol cm(-3). However, in the range from 0.01 to ca. 0.022 mol cm(-3) (0.5 < rho(r) < 1.2), the decreasing slope of the shifts A gradually became smaller and then increased again at the denser region. This anomalous behavior implies the depletion of the local water density around O-2 in the neighborhood of the critical point. (C) 2004 Elsevier B.V. All rights reserved.