It is shown that spin accommodation plays a determining role in the reactivity of aluminum based anion clusters with oxygen. Experimental reactivity studies on aluminum and aluminum - hydrogen clusters show variable reactivity in even electron systems and rapid etching in odd electron systems. The reactivity of even electron clusters is governed by a spin transfer to the singlet cluster through filling of the spin down antibonding orbitals on triplet oxygen. Theoretical investigations show that when the spin transfer cannot occur, the species is unreactive. When spin accommodation is possible, more subtle effects appear, such as the required spin excitation energy, which raises the total energy of the system, and the filling of the antibonding levels of the O-2 molecule, which is stabilized by becoming an aluminum oxygen 7 bond. This explanation is consistent with observed behavior in oxygen etching reactions with a variety of clusters including AI(n)H(m)(-), AI(n),-, AI(n)I(m)(-), and AI(n)C(-). The proposed reaction mechanism lends a physical interpretation as to why the HOMO - LUMO gap successfully predicts oxygen etching behavior of the considered systems.