San Antonio is one of the largest cities in the United States and is in non-attainment of the 8 h ozone standard. Using the Aerodyne Mobile Laboratory, we made observations of ozone and its precursors at three sites in the San Antonio region to determine the main drivers of its production. We found that compounds produced by plants were the dominant organic compound for ozone production and that to limit ozone production at the study site, emissions of nitrogen oxides should be reduced.