A modeling study was undertaken to assess the effect of organic reactivity on photochemical oxidant formation. A six-component hydrocarbon model was developed and tested against data collected in a smog chamber study of irradiated auto exhaust and oxides of nitrogen (NO sub x) mixtures. The model was then adjusted to conditions that are closely approximated those of the urban environment and used to assess the relative reactivity of various organic constituents present in an urban-like air mass. Twelve organics were investigated in the study: ethane, propane, n-butane, ethylene, propylene, trans-2-butene, toluene, m-xylene, methanol, ethanol, formalehyde, and acetaldehyde. The findings of this study indicate that the reactivity of organics depends strongly on the hydrocarbon-to-NO sub x ratio of the mix in which they are reacting. At low hydrocarbon-to-NO sub x ratios, the organics investigated in this study displayed significantly different O sub 3-forming potentials. At high hydrocarbon-to-NO sub x ratios, however, all organics exhibited comparable O to 3-forming potential.