Photochemical decomposition of monoethanolamine (MEA) in the presence of graphitic carbon nitride (g-C3N4) in water under visible-light irradiation was investigated. When aqueous MEA was irradiated by visible light ( >389nm) in the presence of either TiO2 (P25) and O2 or g-C3N4 and argon, virtually no decomposition of MEA was observed. In contrast, when aqueous MEA was irradiated by visible light in the presence of g-C3N4 and O2, the MEA decomposed to produce HCOOH, NO2−, NO3−, and NH3. Although metal (Ag or Pt)-loading onto g-C3N4 enhanced the photocatalytic MEA decomposition slightly, it accelerated the formation of NO2− and NO3−. After the g-C3N4 catalyzed reactions, the recovery of carbon atoms was almost completely accounted for by the remaining MEA and HCOOH formed in the reaction solution, indicating that the carbon atoms in the reacted MEA were predominantly transformed to HCOOH.