Sulphur contamination of alkaline-earth oxide surfaces shows important consequences in many chemistry fields such as surface science and catalysis. We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of SO2 molecules with the bare and electron enriched surfaces of MgO and CaO. Two paramagnetic products were identified via EPR thanks to this interaction, the SO2 center dot- and S2O center dot- radicals whose abundance depends on the surface oxide properties. In particular, higher basicity and higher number of defects in the case of CaO lead to a higher amount of these two radical species. (C) 2010 Elsevier B.V. All rights reserved.