Depending on the localization of hydroxyl groups on MgO surface irregularities, their topology and coordination can differ. In order to discriminate their reactivity, three MgO samples with different morphologies are hydroxylated at various levels. From H-1 MAS NMR three kinds of OH groups can be distinguished. It is shown that the catalytic activity of base sites in the conversion of 2-methylbut-3-yn-2-ol is governed by species giving a H-1 NMR signal at delta < -0.7 ppm, assigned to low coordinated O1CH and O2CH formed by water dissociation on steps and corners. Their reactivity is discussed considering charge and orbital analysis from DFT. (C) 2009 Elsevier Inc. All rights reserved.