Observations of total peroxy radicals (XO<sub>2</sub> = RO<sub>2</sub> + HO<sub>2</sub>) made by the Ethane CHemical AMPlifier (ECHAMP) and concomitant observations of additional trace gases made onboard the Aerodyne Mobile Laboratory (AML) during May 2017 were used to characterize ozone production at three sites in the San Antonio, Texas region. Median daytime [O<sub>3</sub>] was 48 ppbv at the site downwind of central San Antonio. Higher concentrations of NO and XO<sub>2</sub> at the downwind site also led to median daytime ozone production rates (P(O<sub>3</sub>)) of 4.2 ppbv hr<sup>−1</sup>, a factor of two higher than at the two upwind sites. The 95th percentile of P(O<sub>3</sub>) at the upwind site was 15.1 ppbv hr<sup>−1</sup>, significantly lower than values observed in Houston. In situ observations, as well as satellite retrievals of HCHO and NO<sub>3</sub>, suggest that the region is NO<sub><i>x</i></sub> limited for times after approximately 09:00 local time, before which ozone production is VOC-limited. Biogenic volatile organic compounds (VOC) comprised 55 % of total OH reactivity at the downwind site, with alkanes and non-biogenic alkenes responsible for less than 10 % of total OH reactivity in the afternoon, when ozone production was highest. To control ozone formation rates at the three study sites effectively, policy efforts should be directed at reducing NO<sub><i>x</i></sub> emissions. Observations in the urban center of San Antonio are needed to determine whether this policy is true for the entire region.