Abstract: In order to understand the characteristics of initial VOCs (volatile organic compounds), identify their key compounds and source apportionment during a typical ozone (O₃) episode in summer in Huairou, Beijing, a field campaign was conducted on the roof of a building on the Yanqi Lake Campus of the Chinese Academy of Sciences from August 3^rd to 11^th, 2016. The initial mixing ratio of VOCs in the episode was calculated using ratio method and sequential reaction model method. The ozone formation potential (OFP) of the initial VOCs was estimated by the MIR method (maximum incremental reactivity) to identify the key species. And possible sources were examined using the EPA PMF model. The results show that the initial mixing ratio of VOCs was 25.27×10^-9 and the ratio would be underestimated by 18.6% without consideration of VOCs consumed by reactions. The average OFP value was 144.6×10^-9, carbonyls played the most important role in O₃ formation, followed by alkenes, aromatics and alkanes. The key VOC species identified were isoprene, n-hexanal, acetaldehyde, m/p-xylene and toluene. Source analysis shows that vehicle emissions made the largest contribution (23.5%) to the initial VOCs, followed by solvent use (18.3%), biogenic emissions (18.1%), industrial processes (17.6%), biomass burning (12.1%) and combustion emissions (10.5%). Thus, controlling emissions of vehicles nearby, solvent use and industries upwind would reduce VOC levels and controlling the key species such as isoprene, n-hexanal, acetaldehyde, m/p-xylene and toluene would reduce O₃ levels during typical O₃ episodes.