The electronic structures and chemical reactivities (toward H, Li, Li-2, and H-2) are examined for small (MgO)(n) clusters. It is postulated that decreases in the clusters’ reactivity toward the above prototypical species as the cluster size (n) increases is related to the decrease in valence unsaturation that accompanies increases in n. Further, it is suggested that the known high reactivity of MgO powder and crystal is likely related to surface or defect sites that possess high levels of valence unsaturation.