The air–sea-ice CO₂ flux was measured over landfast sea ice in the Chukchi Sea, off Barrow, Alaska, in late May 2008 with a chamber technique. The ice cover transitioned rapidly from a cold winter state to a warm spring state due to a drastic increase in air temperature, also reflected by the onset of surface melt. During melt, brine salinity and brine partial pressure of CO₂ (pCO₂) decreased from 67.3 to 18.7 and 1345.3 to 191.1 μatm, respectively. In contrast, the salinity and pCO₂ of under-ice water at depths of 3 and 5 m below the ice surface remained almost constant with average values of 32.4 ± 0.3 (standard deviation) and 424.8 ± 21.6 μatm, respectively. The air–sea-ice CO₂ flux decreased from +0.7 to –1.0 mmol m^–2 d^–1 (where a positive value indicates CO₂ being released to the atmosphere from the ice surface), driven by the changes in brine pCO₂. During this winter–spring transition, brought on by surface melt, sea ice shifted from a source to a sink for atmospheric CO₂, with brine pCO₂ transitioning from supersaturation to undersaturation relative to the atmosphere. Formation of superimposed ice coincident with melt was not sufficient to shut down ice–air gas exchange.