Included in this report are the results of the first isotopic studies of seawater-dissolved carbon monoxide (CO). Large variations in both carbon and oxygen isotopic compositions (δ13C: -44.7 ~ -21.1?PDB, and δ18O: +8.7 ~ +32.9?SMOW) were observed in seawater-dissolved CO due to the combination of isotopically light CO production in seawater and the enrichment of heavy isotopes during the microbial consumption of CO. Compared with the total δ13C value of dissolved organic matter (DOM), the isotopic compositions of the photochemically produced CO in seawater are depleted in 13C by more than 20?, probably due to large fractionation processes during the photochemical production of CO from DOM. In addition, the isotopic fractionation factors resulting from the microbial oxidation of CO in seawater were estimated to be 1.005 ¿ 0.002 and 1.006 ¿ 0.002 for carbon (αc) and oxygen (αo) based on the incubation experiments. By calculating the weighted mean of the isotopic composition of CO according to the CO concentration observed in surface seawater, the mean δ13C and δ18O for CO emitted from the oceans were estimated to be -40? PDB and +15? SMOW, respectively. Oceanic CO is a unique source of 13C and 18O-depleted CO relative to that of fossil fuel combustion and biomass burning, which will help us create a better isotopic mass balance for both global and local CO budget.