The reactivity of different [Al,20H(2)O](+) Clusters was investigated using gradient-corrected DFT. In clusters with the stoichiometry [AlH(OH)(H2O)(19)](+), which contain a four-fold-coordinated Al-III, H-2 formation was found to occur according to the following mechanism: First a proton is released within the cluster by hydrolysis of a first shell H2O. By successive proton-transfer reactions in the hydrogen-bonded network of the cluster, the proton migrates to the hydridic H at the Al-III cation. Finally, hydride ion and proton recombine to H-2 in an acid-base reaction. The activation barrier for this process was determined to be 45 kJ mol(-1). No H-2 formation was found in hydrated Al-I clusters, [Al-I(H2O)(20)](+).