We here present conductivity-temperature-depth and chemical (CO2, PO4, O2, alkalinity, and pH) observations across an anticyclonic (fresh-core) ring in the Oyashio Current near Bussol Strait, Kuril Islands. These measurements suggest that the core of the ring is a region of enhanced vertical mixing, with attendant effects on both spatial and temporal patterns of CO2 exchange. In particular, the difference between atmospheric and oceanic pCO2 at the air-sea boundary shows a pronounced seasonal reversal in sign. In winter, pCO2 is everywhere higher in the ocean than in the atmosphere; however, because mixing within the core of the ring brings additional amounts of deep, CO2 rich water upward the highest values of pCO2 occur above the ring. In summer, pCO2 outside the ring boundary is also higher in the ocean than in the atmosphere; however, because enhanced mixing within the ring now promotes high values of nutrient flux, primary productivity, and CO2 uptake, pCO2 values there are lower than in the atmosphere. These data suggest the importance of including mesoscale physical processes in the interpretation of oceanic CO2 exchange and demonstrate the usefulness of biogeochemical properties in studies of ring dynamics and evolution. ¿ American Geophysical Union 1996