Physisorption of N-2 gas onto the surface of a metal oxide (MgO or CaO), containing paramagnetic trapped electron centers (F-s(+) color centers), leads to the formation of a paramagnetic species that, on the basis of its EPR spectrum and of the related spin-Hamiltonian parameters, is identified as a N-2(-) radical anion. The species in fact contains two nitrogen atoms and its g and A tensors are in agreement with what observed for the N-2(-) radical trapped in irradiated crystal of various azides. The surface N-2 species is formed by surface-to-molecule one-electron transfer, and its stability strictly parallels the stability of the physisorbed layer, the species formation being completely reversible and pressure dependent. When the N-2 adlayer is desorbed, in fact, the N-2(-) spectrum vanishes and the original F-s(+) spectrum is restored. Ab initio quantum chemical calculations on an embedded MgO cluster fully confirm the observed phenomenon indicating, in agreement with EPR analysis, the electron transfer of a large fraction of electron density into the pi orbitals of the admolecule.