Aiming at an estimate of the number of chemically active surface defects on MgO nanocrystals, we used Li+ doping in conjunction with subsequent thermal annealing. Changes in the surface reactivity of the Li+-doped nanocrystals were monitored by IR and electron paramagnetic resonance spectroscopy using chemisorbed hydrogen, surface trapped electrons, and surface complexed O-2 as molecular probes. It was found that the admixture of 0.2 at% Li+ not only significantly reduces the thermal stability, but also changes all surface spectroscopic features specific to pure MgO. On the basis of the Li+ doping effect, the maximum concentration of active surface sites on undoped MgO nanocrystals corresponds to 3% of a nanocrystal monolayer. (c) 2007 Elsevier Inc. All rights reserved.