Soybeans were grown for three seasons in open-top field chambers to determine (1) whether elevated CO2 (360 versus 700 μmol mol−1) alleviates some of the yield loss due to pollutant O3, (2) whether the partial stomatal closure resulting from chronic O3 exposure (charcoal-filtered air versus 1.5 × ambient concentrations) is a cause or result of decreased photosynthesis, and (3) possible implications of CO2/O3 interactions to climate change studies using elevated CO2. Leaf conductance was reduced by elevated CO2, regardless of O3 level, or by exposure to O3 alone. As.a result of these effects on conductance, high CO2 reduced estimated midday O3 flux into the leaf by an average of 50% in charcoal-filtered air and 35% in the high O3 treatment. However, while exposure to O3 reduced seed yields by 41% at ambient CO2 levels, the yield reduction was completely ameliorated by elevated CO2. The threshold midday O3 flux for yield loss appears to be 20–30 nmol m−2 s−1 in this study. Although elevated CO2 increased total biomass production, it did not increase seed yields. A/Ci curves show a large reduction in the stomatal limitation to photosynthesis due to elevated CO2 but no effect of O3. These data demonstrate that (1) reduced conductance due to O3 is the result, and not the cause, of reduced photosynthesis, (2) 700 μmol mol−1 CO2 can completely ameliorate yield losses due to O3 within the limits of these experiments, and (3) some reports of increased yields under elevated CO2 treatments may, at least in part, reflect the amelioration of unrecognized suppression of yield by O3 or other stresses.