Effects of pressure on lattice parameters and molar volume of diaspore were studied to 25.5 GPa at 300 K by polycrystailine X-ray diffraction techniques. The pressure-volume data fit with a second-order Birch-Murnaghan equation of state give a bulk modulus K0 = 167.5 GPa. The bulk modulus is twice the previously reported, but unsubstantiated, value of K0 = 85 GPa by Ruoff and Vanderborgh [Phys. Rev. Lett. 66, 754 (1991)]. Thermodynamic calculation indicates that hydrogen and ruby are stable relative to diaspore and aluminum to pressures above 200 GPa.