A field campaign was carried out in September–November 2010 near the summit of Mt. Tai Mo Shan in Hong Kong. Isoprene, methyl vinyl ketone (MVK) and methacrolein (MAC) were measured. The average isoprene mixing ratio was 109 pptv, and the average MAC and MVK levels were 68 pptv and 164 pptv, respectively. The average daytime levels of isoprene (149 ± 20 pptv, average ± 95% confidence interval, p < 0.01), MAC (70 ± 9 pptv, p < 0.01) and MVK (169 ± 22 pptv, p < 0.1) were significantly higher than the average nighttime values (20 ± 5 pptv, 49 ± 8 pptv and 139 ± 25 pptv, respectively). The relationship between MVK and MAC indicated that nearby isoprene oxidation dominated their daytime abundances, while NO3 chemistry and regional transport of anthropogenic sources from inland Pearl River Delta region could explain the higher MVK to MAC ratios at night. Correlation analysis of [MVK]/[isoprene] versus [MAC]/[isoprene] found that the isoprene photochemical ages were between 10 and 64 min. Regression analysis of total O3 (O3 + NO2) versus MVK resulted in an estimated contribution of isoprene oxidation to ozone production of 12.5%, consistent with the simulated contribution of 10-11% by an observation-based model.