Co/SiO2, Mg-Co/SiO2, and Co-Mg/SiO2 catalysts were prepared from nitrate precursors to get more insight into the effect of magnesium promotion on the cobalt catalyst. The desorption characteristics and reactivity of the catalysts towards synthesis gas were evaluated in a pulse micro reactor connected to an on-line quadrupole mass spectrometer. The presence of MgO both decreased the extent of reduction and increased the dispersion of cobalt. The reactivity results suggested that MgO promotion created new types of active sites, probably at the edge sites of cobalt and magnesium. The highest activity per metallic site available was obtained with a Mg : Co molar ratio of 1/2, i.e. with a high amount of edge sites. In accordance with previous results the formation of CO2 was clearly suppressed in the presence of MgO, and methanol was formed in trace amounts.