The concentration of carbon monoxide () in first-year ice in Franklin Bay, southeastern Beaufort Sea, was ~40 times higher than in the underlying seawater and ~15 times higher than in the adjacent open water. The in the sea ice decreased with increasing depth and increased rapidly at the bottom where there was an abundance of ice microalgae. The depth distribution of was consistent with a photochemical source of this compound in sea ice, which was further inferred from the vertical profiles of the dissolved organic matter absorption coefficients and directly verified by incubation of ice samples refrozen from melted ice. Results from this study suggest that substantial photooxidation of organic matter occurs in sea ice. This process may affect the organic carbon cycle in the Arctic Ocean. The potential flux of CO from sea ice to the Arctic atmosphere is estimated to be 1.4 ¿ 1010 moles a-1.