An argon hydrate (AH) single crystal was synthesized in a diamond anvil cell to investigate its intrinsic high-pressure properties by the optical microscopy and in situ Raman scattering measurements. Single crystalline AH at room temperature showed three phase transitions at P = 0.43, 0.66, and 1.05 GPa through the changes in crystalline morphology or color and in Raman spectra of lattice vibrational modes and O-H stretching vibrations of H2O host cages. Argon multi-occupancies of large cages in the initial sII and higher-pressure sH and sT phases are investigated by Raman bands observed around 135 cm(-1) in view of recent MD calculation, and neutron and X-ray diffraction experiments. (C) 2002 Elsevier Science B.V. All rights reserved.