The　adsorption　and　dissociation　of　Cl2　at　different　magnesium　vacancy　sites　(V,　V-　or　V2-　center)　of　the　MgO　(001)　defect　surface　have　been　investigated　using　density　functional　theory　coupled　with　embedded　cluster　models.　It　is　rather　interesting　to　find　that　the　magnesium　vacancy　sites　at　the　edge　or　corner　exhibit　quite　high　surface　reactivity　toward　the　dissociation　of　Cl2.　The　dissociation　process　proceeds　without　energy　barrier　as　reflected　by　the　calculated　potential　energy　curve.　Whereas,　the　case　is　different　for　the　magnesium　vacancy　sites　at　the　terrace,　i.e.,　no　surface　reactivity　toward　the　dissociation　of　Cl2.　Meanwhile,　the　results　herein　are　compared　to　those　at　＂electron-rich＂　oxygen　vacancy　sites.　The　present　findings　illustrate　that　the　low-coordinated　magnesium　vacancy　sites　on　the　MgO　(001)　surface　should　be　attached　to　importance　in　the　catalysis　community　because　they　can　act　as　active　centres　in　adsorption　and　dissociation　processes　of　technological　importance.　We　believe　that　these　results　would　be　particularly　beneficial　for　understanding　the　chemical　reactivity　of　MgO　defect　surfaces.　©　2005　Elsevier　B.V.　All　rights　reserved.