UV absorption spectra of molecular oxygen were measured at pressure up to 1.5 GPa and temperature between 17 and 297 K. In the supercritical phase, an absorption band was observed in a UV region between 4.5 and 7 eV and showed an appreciable enhancement in the intensity with pressure. The absorption cross-section, sigma(eff), of the UV band at 1.5 GPa, estimated from the molar volume obtained by present X-ray diffraction experiments, reached three orders of magnitude of previous one measured at 1.0 MPa. The observed UV absorption of high-density oxygen was attributed to the two-molecule process by an antiferromagnetic O-2 pair. (C) 2004 Elsevier B.V. All rights reserved.