Powder X-ray diffraction patterns were obtained with a Philips X’PERT diffractometer (Cu KR radiation, 2 kW, with an X’Celerator RTMS detector, automatic divergence slit). The powder was measured on a Si single crystal and sealed in the glovebox by an airtight hood made of Kapton foil, the foil being mounted out of the focus of the spectrometer. The software for data acquisition and evaluation was X’PERT 1.3e and ProFit 1.0c. PowderCell Vers. 2.4 was used to calculate the powder diffraction pattern of Ca(AlH4)2‚4THF from the single-crystal data.