Hydrated aluminum cluster cations Al+(HO2)(n) with n=3-50 were stored in a collision-free environment, and their unimolecular fragmentation induced by the room temperature "black body" infrared background was studied. An interesting size-dependent intracluster reaction resulting in oxidation of the aluminum cation and elimination of molecular hydrogen was observed. The results are discussed and compared with similar recent studies of hydrated magnesium clusters.