The effect of H2S upon the acidity of the MgO surface has been investigated using FTIR spectra of adsorbed dimethylpyridine (2,6-DMP) and CO as test molecules. In the presence of weakly bound H2S, the spectra of 2,6-DMP reveal Brønsted acidity. Adsorption of CO perturbs OH groups from MgO only in the presence of H2S or of aprotic molecules such as CO2 or SO2. This infers that these are the surface OH groups activated by acidic molecules which act as the source of protons. Thus, weak adsorption of acidic compounds, such as H2S, SO2, or CO2, can lead to dramatic enhancement of the acidity of the MgO surface. © 2009 American Chemical Society.