Surface anions on edges (4-coordinated = 4C) and on corners (3-coordinated = 3C) of cubic MgO nanoparticles exhibit UV resonance absorptions around 5.5 and 4.6 eV, respectively. After monochromatic excitation of either site the electron paramagnetic resonance (EPR) spectrum exhibits exclusively signal components related to 3-coordinated O- radicals (O-(3C), electron hole centers), which are perfectly bleached by H(2) addition. The disappearance of the O-(3C) EPR signal components is paralleled by a depletion of the UV resonance absorption of the 3-coordinated O(2-) only and the appearance of one single band in the OH stretching region of the IR spectrum. Obviously the sites of UV excitation and subsequent UV induced surface reaction with H(2) are not the same. This may coherently be explained in terms of mobility of the exciton (O(2-)(4C)* or--after ionization--of the corresponding electron hole O-(4C) along the edge where it was created. Finally the mobile state is trapped at a corner site where the O(3C)H group is formed.