Ozone production efficiency can be defined as the number of molecules of oxidant (O3 + NO2) produced photochemically when a molecule of NOx (NO + NO2) is oxidized. It conveys information about the conditions under which O3 is formed and is an important parameter to consider when evaluating impacts from NOx emission sources. We present calculational and observational results on ozone production efficiency based on measurements made from aircraft flights in the Phoenix metropolitan area in May and June of 1998. Constrained steady state box model calculations are used to relate a ratio of O3 production rate to NOx consumption rate (i.e., P(O3)/P(NOz)) to a VOC to NO2 ratio of OH reactivity. Lagrangian calculations show how this ratio generally increases with time due to oxidation chemistry and plume dilution. City to city differences in ozone production efficiency can be attributed to corresponding differences in VOC to NO2 reactivity ratio which in turn reflect emission patterns. Ozone production efficiencies derived from aircraft measurements in 20 plumes show a dependence on NOx concentration similar to that calculated for P(O3)/P(NOz). Calculations are based on data from a single location but are believed to be applicable to a wide range of plumes from different areas.