A sterically hindered water-soluble porphyrin, tetrakis(3-sulfonatomesityl)porphyrin (H2TSMP), could form stable radical cation in aqueous media after electrochemical one electron oxidation. The anodic oxidation of H2TSMP in the presence of tripropylamine or C2O42- as a coreactant in aqueous solution produces electrogenerated chemiluminescence (ECL) with maxima at 640 and 700 nm. The same emission spectrum of ECL and fluorescence indicates that the ECL emission is from the singlet state of H2TSMP. The annihilation reactions of ZnTSMP·+ and ZnTSMP·-, which are generated electrochemically, in CH3CN + H2O (1:1) mixed solution results in an emission which is identical to the photoluminescence. Protection of the active sites against the nucleophilic attack of water or OH- by sterical hinderance is a successful strategy in designing new ECL-active compounds in aqueous media. Both ECL reaction mechanisms are proposed.