<p><strong>Abstract.</strong> Here, we present an assessment of long-term trends in O<sub>3</sub> and odd oxygen (O<sub>3</sub> + NO<sub>2</sub>) at the industrial Monterrey metropolitan area (MMA) in NE Mexico. Diurnal amplitudes in O<sub><i>x</i></sub> (AV<sub>d</sub>) are used as a proxy for net O<sub>3</sub> production, which is influenced by the NO<sub>2</sub> photolysis rate. No significant differences in the AV<sub>d</sub> are observed between weekends and weekdays, although the largest AV<sub>d</sub> values are observed at sites downwind of industrial areas. The highest O<sub>3</sub> mixing ratios are observed in spring, with minimum values in winter. The largest annual variations in O<sub>3</sub> are typically observed downwind of the MMA, with the lowest variations generally recorded in highly populated areas and close to industrial areas. A wind sector analysis of mixing ratios of O<sub>3</sub> precursors revealed that the dominant sources of emissions are located in the industrial regions within the MMA and surrounding area. Significant increasing trends in O<sub>3</sub> in spring, summer, and autumn are observed depending on site location, with trends in annual averages ranging between 0.19 and 0.33<span class="thinspace"></span>ppb yr<sup>−1</sup>. Overall, from 1993 to 2014, within the MMA, O<sub>3</sub> has increased at an average rate of 0.22<span class="thinspace"></span>ppb yr<sup>−1</sup> (<i>p</i> &lt; 0. 01), which is in marked contrast with the decline of 1.15<span class="thinspace"></span>ppb yr<sup>−1</sup> (<i>p</i> &lt; 0. 001) observed in the Mexico City metropolitan area (MCMA) for the same period. No clear trend is observed from 1996 to 2014 within the Guadalajara metropolitan area (GMA).</p>