Correlations are presented between model predictions for O3-NOx-hydrocarbon sensitivity and afternoon concentrations of four ''indicator species'': NOy, O3/(NOy-NOx), HCNO/NOy, and H2O2/HNO3. The indicator species correlations are based on a series of photochemical simulations with varying rates of anthropogenic and biogenic emissions and meteorology. Hydrocarbon-sensitive chemistry in models is shown to be linked to afternoon NOy>20 ppb, O3/(NOy-NOx)<7, HCHO/NOy<0.28, and H2O2/HNO3<0.4. Lower NOy and higher ratios correspond with NOx-sensitive ozone. The correlation between NOx-hydrocarbon sensitivity and indicator species remains, even when model emission rates and hydrocarbon/NOx ratios are changed by a factor of 2. Methods are developed for evaluating the goodness of fit between model NOx-hydrocarbon sensitivity and indicator values. Ozone chemistry is also analyzed in terms of fundamental properties of odd hydrogen, and theoretical criteria for the transition between NOx- and hydrocarbon-sensitive regimes are derived. A theoretical correlation between O3 and H2O2+NOy-NOx is developed as a way to extend rural O3-NOy correlations into urban locations. Measured indicator values during pollution events in Los Angeles, Atlanta, and rural Virginia are used to illustrate the range of observed values under different environmental conditions. ¿ American Geophysical Union 1995