The g-tensor of excess electron centres (H+)(e(-)) generated at the surface of polycrystalline MgO has been determined by multi-frequency, high-field electron-paramagnetic resonance (EPR) performed at 9.5, 34, 190 and 285 GHz. The A tensor for the interacting H-1 was also determined by complementary Q-band electron-nuclear double resonance (ENDOR). Due to the high spectral resolution the anisotropy of the g-tensor could be fully resolved in the powder continuous-wave EPR spectrum. Computer simulation of the experimental data revealed small g-tensor anisotropies: Delta g(x) = -0.00294, Delta g(y) = -0.00286, Delta g(z) = -0.00101, with Delta g(i) = g(i) - 2.0023. (c) 2007 Elsevier B.V. All rights reserved.