HKUST-1 is a strictly microporous crystalline metal organic framework with pore sizes of 5, 11, and 13.5 angstrom. Detailed gas adsorption measurements show that its adsorption capacity for water at 20 degrees C is higher than that for nitrogen at -196 degrees C, and far exceeds that for methane at 0 degrees C. Extended exposure to water vapour at high relative humidity, or consecutive adsorption-desorption cycling of water vapour, destroys both the MOF crystal structure and its adsorption capacity, after a reduced number (< 5) of cycles. Destruction proceeds through mesoporous defects that open within the crystal structure, as attested both by the development of hysteresis in the adsorption isotherms and by changes in the small angle X-ray scattering pattern. In the pristine crystal, the structure of the water in the micropores closely resembles that of bulk liquid water. Small angle neutron scattering demonstrates that water is adsorbed preferentially over methane, and that the size of the spherical cavities occupied by the adsorbed water molecules in the intact crystal is consistent with the known pore size structure in this system.