We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of SO2 molecules on bare and partially hydroxylated alkali-earth oxide (AEO) surface. When SO2 is adsorbed on the surfaces of higher basicity (CaO and SrO) disproportionation reaction occurs with the formation of SO2 center dot- radical. We show also that, when the same reaction is present on partially hydroxylated surfaces a fraction of the electrons exchanged during the redox process are trapped on the oxide surface forming colour centres ((H+)(e(-)) centres). (C) 2011 Elsevier B.V. All rights reserved.