The electron affinities of beryllium and magnesium tetramers are calculated at the ROMP2 level employing the Dunning-type aug-cc-pVQZ basis set. The vertical electron detachment energy (VEDE) amounts to 1.685 eV for Be4- and 0.943 eV for Mg4 . The decomposition of the VEDE into physical components and an atomic orbital population analysis are used to elucidate the nature of the outer electron binding in these anions. [Figure: see text]. The lowest unoccupied molecular orbitals in the ground state of Mg4 : a LUMO, symmetry A1, b LUMO + 1, symmetry T2; c the highest occupied molecular orbital (HOMO), symmetry A1 in the ground state of Mg4-.