Airborne surveillance of gas emissions from Augustine Volcano and other Cook Inlet volcanoes began in 1990 to identify baseline emission levels during noneruptive conditions. Gas measurements at Augustine for SO₂, CO₂, and H₂S showed essentially no evidence of anomalous degassing through spring 2005. Neither did a measurement on May 10, 2005, right after the onset of low level seismicity and inflation. The following measurement, on December 20, 2005, showed Augustine to be degassing about 600 metric tons per day (t/d) of SO₂, and by January 4, 2006, only 7 days before the first explosive event, SO₂ emissions had climbed to ten times that amount. Maximum emission rates measured during the subsequent eruption were: 8,930 t/d SO₂ (February 24, 2006), 1,800 t/d CO₂ (March 9, 2006), and 4.3 t/d H₂S (January 19, 2006). In total, 45 measurements for SO₂ were made from December 2005 through the end of 2008, with 19 each for CO₂ and H₂S during the same period. Molar CO₂/SO₂ ratios averaged about 1.6. In general, SO₂ emissions appeared to increase during inflation of the volcanic edifice, whereas CO₂ emissions were at their highest during the period of deflation associated with the vigorous effusive phase of the eruption in March. High SO₂ was probably associated with degassing of shallow magma, whereas high CO₂ likely reflected deep (>4 km) magma recharge of the sub-volcanic plumbing system, For the 2005–6 period, the volcano released a total of about 1.5×10⁶ tons of CO₂ to the atmosphere, a level similar to the annual output of a medium-sized natural-gas-fired powerplant. Augustine also emitted about 8×10⁵ tons of SO₂, similar to that produced by the 1976 and 1986 eruptions of the volcano.