Attached soybean leaves in a closed system were fed high specific activity 14CO2 at four CO2 compensation points (30, 53, 110, 204 μl/l CO2) which were produced by different O2 concentrations (10, 21, 44, 80% O2). Rates of CO2 uptake and evolution were measured simultaneously using an infrared CO2 gas analyzer and a Geiger tube. The CO2 compensation point was proportional to O2 concentration but did not change for the two light intensities used. The rates of CO2 exchange at the compensation point did increase with light intensity. These rates of CO2 exchange also increased with the increasing CO2 concentration of the compensation point (for increasing O2 concentrations) but tended to saturate for compensation points greater than 110 μl/l CO2.