Different MgO samples were investigated by means of X-ray photoelectron spectroscopy and catalytic, desorption and basicity measurements. The O 1s peak of MgO showed a shoulder (or was split into two peaks) as a function of storage time. This shoulder and the second peak were identified as oxygen atoms of Mg(OH)2 with a binding energy of 531.5 ± 0.3 eV, compared with 529.9 ± 0.3 eV for pure MgO. In desorption measurements, only one H2O peak at ≈ 373 K was found with a freshly prepared MgO sample, whereas the other samples showed two peaks, one at ≈ 373, the other at 573 K. These experiments demonstrated the formation of a hydroxide layer on MgO with increasing storage time. With the growth of the hydroxide layer the basicity of the MgO samples decreased. The dehydrogenation and dehydration of butan-2-ol was used as test reaction. With increasing hydroxide layer and decreasing basicity the selectivity changed from dehydrogenation to dehydration.