Abstract: Data collected from 2006-08-10 to 2006-09-18 were analyzed to study the photochemical pollution in Wuqing District, Tianjin. 26 average hourly values of ozone exceeded the national air quality standard during the monitoring periodand accounted for 2.7% of observed values. When photochemical pollution occurred, ρ(NO_x) and activity of volatile organic compounds (VOCs) increased; ρ(NO₂) increased by 20 μg/m³ and VOCs increased by 42%. However, O₃ was even more sensitive to increased ρ(NO_x). o-Xylene was the largest contributor to ozone formation according to equivalent propylene concentrations of hydrocarbons. The most reactive category in hydrocarbons were alkenes, then aromatic hydrocarbons and alkanes.