The structure of argon hydrate was studied at ambient pressure and low temperature, and between 1.7 and 4.2 GPa at 295 K. This analysis produced a single Ar guest atom, positionally disordered off-center in the large cages of sII. Above 1.7 GPa Ar clathrate transformed to a mixture of a body-centered orthorhombic filled-ice phase, which can be viewed as a polytype of ice-Ih, and high pressure forms of pure ice. The guest disorder is further substantiated by analysis of the guest to host ratio in this high pressure filled-ice structure. The bulk modulus of Ar filled-ice found to be 11.7 0.4 GPa. (C) 2009 Elsevier B. V. All rights reserved.