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Measurements of atmospheric O-2 have been used to quantify large-scale fluxes of carbon between the oceans, atmosphere and land since 1992 (Keeling and Shertz, 1992). With time, datasets have grown and estimates of fluxes have become more precise, but a key uncertainty in these calculations is the exchange ratio of O-2 and CO2 associated with the net land carbon sink (alpha(B)). We present measurements of atmospheric O-2 and CO2 collected over a 6-year period from a mixed deciduous forest in central Massachusetts, USA (42.537 degrees N, 72.171 degrees W). Using a differential fuel-cell-based instrument for O-2 and a nondispersive infrared analyzer for CO2, we analyzed airstreams collected within and similar to 5 m above the forest canopy. Averaged over the entire period of record, we find these two species covary with a slope of -1.081 +/- 0.007 mol of O-2 per mole of CO2 (the mean and standard error of 6 h periods). If we limit the data to values collected on summer days within the canopy, the slope is 1.03 +/- 0.01. These are the conditions in which biotic influences are most likely to dominate. This result is significantly different from the value of -1.1 widely used in O-2-based calculations of the global carbon budget, suggesting the need for a deeper understanding of the exchange ratios of the various fluxes and pools comprising the net sink.