Electrogenerated　chemiluminescence　(ECL)　of　nanoparticles　are　known　to　be　strongly　dependent　on　the　surface　states,　whereas　the　detailed　mechanisms　are　not　clear.　Here,　the　detailed　ECL　mechanisms　of　CdS　nanopaticles　have　been　investigated.　It　was　found　that　ECL　emission　observed　at　-0.95　V　(600　nm,　700　nm)　is　assigned　to　the　reduction　of　the　surface　S　vacancy,　whereas　the　luminescence　at　-1.25　V　(520　nm)　is　attributed　to　the　electron　injection　to　the　conduction　band　in　the　cathodic　process.　On　the　basis　of　these　mechanisms,　for　the　first　time,　we　are　able　to　observe　the　gradual　ECL　evolution　of　CdS　nanoparticles　in　the　presence　of　oxidant,　therefore　online　monitoring　the　surface　oxidation　of　CdS　nanoparticles　by　the　ECL　method.　This　method　will　find　more　application　in　surface　probing　of　nanomaterials　because　the　ECL　of　nanomaterials　is　very　sensitive　to　the　surface.　©　2011　American　Chemical　Society.