Abstract: Ambient volatile organic compound (VOCs) samples were collected from three sites (urban area, Zhenhai and Beilun) in Ningbo during 2010. A total of 36 VOC species were determined. The chemical composition and spatiotemporal variations of ambient VOCs were investigated. Key reactive species of VOCs were identified based upon OH radical reaction rate (LOH). The average mass concentration of the total VOCs (sum of 36 VOCs) was 198.2 μg/m³, and the major components were alkanes (48.6%), aromatics (33.6%) and alkenes (17.8%). Seasonal concentrations of the total VOCs (ρ(VOCs)) were ordered as winter (298.5 μg/m³) > autumn (174.1 μg/m³) > spring (122.0 μg/m³). ρ(VOCs) in the urban area (161.3 μg/m³) was lower than those in Zhenhai (225.0 μg/m³) and Beilun (208.2 μg/m³). The relative compositions of the total VOCs were stable. Having the similar LOH to ethylene, the chemical reactivity of total VOCs was relatively strong. Alkenes, being only 22% of the mixing ratio of VOCs, contributed the most to LOH and accounted for over 64% of chemical reactivity. The key reactive species of ambient VOCs were 1-butene, trans-2-butene, m,p-xylene, ethylene and pentene.