Résumé : Although salt rejection from sea ice is a key process in deep-water formation in ice-covered seas, the concurrent rejection of CO2 and the subsequent effect on air-sea CO2 exchange have received little attention. We review the mechanisms by which sea ice directly and indirectly controls the air-sea CO2 exchange and use recent measurements of inorganic carbon compounds in bulk sea ice to estimate that oceanic CO2 uptake during the seasonal cycle of sea-ice growth and decay in ice-covered oceanic regions equals almost half of the net atmospheric CO2 uptake in ice-free polar seas. This sea-ice driven CO2 uptake has not been considered so far in estimates of global oceanic CO2 uptake. Net CO2 uptake in sea-ice-covered oceans can be driven by; (1) rejection during sea-ice formation and sinking of CO2-rich brine into intermediate and abyssal oceanic water masses, (2) blocking of air-sea CO2 exchange during winter, and (3) release of CO2-depleted melt water with excess total alkalinity during sea-ice decay and (4) biological CO2 drawdown during primary production in sea ice and surface oceanic waters. © 2011 The Authors Tellus B © 2011 John Wiley & Sons A/S.