In the course of heterogeneous catalytic transfer hydrogenation (CTH) of carbonyl compounds with alcohols on MgO, the catalyst is continuously poisoned, which leads to a complete loss of activity. The main result of the present work is that deactivation may be prevented by treatment with chloroform and the initial activity of MgO is retained for as long as 65 h. Detailed investigations were carried out (activations of MgO, TG, DSC, chloroform treatment, FT-IR spectroscopy, CDCl3 adsorption) in order to identify both types of active centers responsible for poisoning and CTH. Based on our own experimental results as well as on data in the literature, the prerequisites of CTH on MgO appear to be basic (O2-) site and adjacent surface hydroxyl site pairs. Lewis acid centers responsible for poisoning are blocked and active surface hydroxyl groups of weakly acidic character are simultaneously generated by chloroform treatment.