Ambient air pollutants data were used to evaluate whether O3 formation at a specific site (Sirifort) in Delhi was limited by volatile organic compounds (VOCs) or oxides of nitrogen (NOx). For this purpose, a photochemical model OZIPR (Ozone Isopleth Plotting Research) based on Lagrangian trajectory model was applied to 9 ozone episodes that occurred at Sirifort from August to October 2006. Emissions data were estimated using an area-source box model. The results show that the prediction for peak O3 concentration agreed reasonably well with the observed data. O3-isopleth plots clearly reveal that O3 formation is more sensitive to VOC emissions for lower VOCs/NOx ratio, while for higher VOCs/NOx ratio, O3 formation is more sensitive to NOx emissions. However, for the purpose of practical O3 control applications at the observation site, it is concluded that VOCs emissions should be reduced while keeping a lower VOCs/NOx ratio.