Antioxidant metabolism is responsive to environmental conditions, and is proposed to be a key component of ozone (O3) tolerance in plants. Tropospheric O3 concentration ([O3]) has doubled since the Industrial Revolution and will increase further if precursor emissions rise as expected over this century. Additionally, atmospheric CO2 concentration ([CO2]) is increasing at an unprecedented rate and will surpass 550 ppm by 2050. This study investigated the molecular, biochemical and physiological changes in soybean exposed to elevated [O3] in a background of ambient [CO2] and elevated [CO2] in the field. Previously, it has been difficult to demonstrate any link between antioxidant defences and O3 stress under field conditions. However, this study used principle components analysis to separate variability in [O3] from variability in other environmental conditions (temperature, light and relative humidity). Subsequent analysis of covariance determined that soybean antioxidant metabolism increased with increasing [O3], in both ambient and elevated [CO2]. The transcriptional response was dampened at elevated [CO2], consistent with lower stomatal conductance and lower O3 flux into leaves. Energetically expensive increases in antioxidant metabolism and tetrapyrrole synthesis at elevated [O3] were associated with greater transcript levels of enzymes involved in respiratory metabolism.