The nature of electron traps at the surface of polycrystalline MgO is analyzed for the first time in terms of interaction of the electron spin with the nuclear spin of the O-17 anions of the surface. MgO crystals enriched in the O-17 isotope have been prepared and the corresponding hyperfine coupling constants have been measured in EPR spectra. The trapped electrons are produced by exposure of the samples to H-2 followed by UV irradiation, with consequent production of paramagnetic (H+)(e(-)) centers. The EPR spectrum shows a multiplet structure with two main sextets separated by 51 and 10 G, respectively, due to the interaction of the unpaired electron with two nonequivalent O-17 nuclei. The results are interpreted based on accurate quantum chemical calculations. It is suggested that the paramagnetic centers observed correspond to (H+)(e(-)) pairs formed at step sites of the MgO surface.