The thermal decomposition of alane was investigated by application of synchrotron X-ray diffraction (SR-XRD) and thermal desorption spectroscopy (TDS). Two polymorphs were studied, α- and γ-AlH3. Activation energies, anisotropic volume expansions, and phase transformation paths were found. In addition, the crystal structure data, including structure of hydrogen sublattice, and small charge transfer from the aluminium towards the hydrogen sites were observed during a high-resolution SR-XRD study of α-AlH3.