An aluminium tetrahydride species has been formed after electronic excitation at about 5 eV of A1H(2)(H-2) van der Waals aggregates isolated in argon matrices. Isotopic studies characterized the five isotopic species (corresponding to -H-4, -H3D, -H2D2, -HD3 and -D-4 containing molecules) and demonstrated that this species has four equivalent A1-H bonds with a tetrahedral or very nearly tetrahedral structure. Ab initio studies using the coupled-cluster singles and doubles [CCSD (T,E4T)] and density functional theory (DFT) methods confirmed the highly unstable character of an hypothetical neutral A1H, species as previously established by other workers, which would spontaneously dissociate back into A1H(2)(H-2), On the other hand, the calculated vibrational spectrum of the tetrahydroaluminate anion, A1H(4)(-), would perfectly fit that of the observed species. Identification of the isolated A1H(4)(-) is thus proposed.