Tropospheric concentrations of carbon dioxide ([CO2]) and ozone ([O3]) are increasing as a result of land-use changes and fossil fuel combustion. These changes are hypothesized to have a large effect on water use for crops. While leaf-level responses to elevated [CO2] and [O3] are well documented, few studies have addressed canopy responses to increases in these pollutants. Evapotransporation (ET) for soybean was measured using an energy balance approach. Plots were exposed to a 180 µmol mol-1 increase in [CO2], a 25% increase in ambient [O3], or both. Data for elevated [CO2] and elevated [O3] were collected throughout the 2002-2005 growing seasons and the elevated [CO2]+[O3] treatment was added in 2003. Micrometeorological measurements were recorded in 10-minute intervals throughout the season for each of the treatments. Sensible heat flux was calculated from the wind and temperature measurements. ET was determined as a residual in the energy balance from net radiation, sensible heat flux, and soil heat flux. The results show a general decrease in ET for all three treatments with the largest decrease observed for the plants grown in elevated [O3] during most years. When integrated over the entire growing season for 2002 and 2003, plants grown in elevated [CO2] and elevated [O3] used 10 and 18% less water, respectively. Elevated [CO2] showed a greater decrease in 2004 and 2005 than 2002 or 2003 which is consistent with a larger decrease in leaf level stomatal conductance measured in 2004. During 2004, when ambient [O3] was lower, there was no effect of increases in [O3] on ET. While the effect of these treatments on ET are lower than observed for leaf stomatal conductance, periods with a large treatment- induced decrease in stomatal conductance result in larger decreases in ET, suggesting that stomatal conductance is strongly involved in the observed decreases in ET.