Ozone isopleths plotted with measured values of ozone, hydrocarbons and total reactive nitrogen (NOy) are proposed to visualize ozone production regimes at distinct locations. The applicability of the concept is confirmed with measurements at two distinct sites in the Italian Po Basin. Empirical ozone isopleths at the urban site clearly show ROG-sensitive ozone production, with ozone increasing with increasing ROG concentrations and decreasing with increasing ambient levels of NOx. At the rural site NOx-sensitive ozone production prevails, in accordance with previous results. ¿ 2001 American Geophysical Union