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High ozone (O-3) concentrations are a major concern about air quality in the Sao Paulo Metropolitan Area (SPMA). During 2016, the 8-hour state standard of 140 mu g m(-3) was exceeded on 32 days, whereas the 1-hour national standard of 160 mu g m(-3) was exceeded on 76 days. Exposure to such unhealthy O-3 levels and other pollutants can lead to respiratory disease. The focus of this study is to determine the main O-3 precursor in terms of the volatile organic compounds (VOCs) in order to provide a scientific basis for controlling this pollutant. In this work, 66 samples of hydrocarbons, 62 of aldehydes and 42 of ethanol were taken during the period from September 2011 to August 2012 from 7:00 to 9:00 a.m. The OZIPR trajectory model and SAPRC atmospheric chemical mechanism were used to determine the major O-3 precursors. During the studied period, aldehydes represented 35.3% of the VOCs, followed by ethanol (22.6%), aromatic compounds (15.7%), alkanes (13.5%), ketones (6.8%), alkenes (6.0%) and alkadienes (less than 0.1%). Considering the concentration of VOCs and their typical reactivity, the simulation results showed that acetaldehyde contributed 61.2% of the O-3 formation. The total aldehydes contributed 74%, followed by aromatics (14.5%), alkenes (10.2%), alkanes (1.3%) and alkadienes (e.g., isoprene; 0.03%). Simulation results for the SPMA showed that the most effective alternative for limiting the O-3 levels was reducing the VOC emissions, mainly the aldehydes.