We report recent measurements of ozone (O3) and related trace gases obtained in June--July 2005 in a mountainous area north of Beijing. Of the 39 days of observation, there were 13 days whose 1-hr O3 mixing ratio exceeded 120 ppbv, with a maximum level of 286 ppbv, which is the highest reported value in open literature for China. Analysis of plume characteristics and air-mass back trajectories show the highest concentrations of O3 (and other trace gases) were mainly due to emissions from the Beijing urban area. O3 was strongly correlated with NOy with O3-NOy regression slopes of 3--6 ppbv/ppbv in the six episodes examined. In contrast to some observations from North America, the positive O3-NOy correlation in the Beijing plumes extended to NOy mixing ratios up to 55 ppbv, suggesting an important role of NOx in the formation of O3 in Beijing. The very high concentrations of O3 revealed in our study imply significant negative effects on vegetation and regional air quality.