Isoprene, monoterpenes, β-caryophyllene, and toluene are known to be important secondary organic aerosol (SOA) precursors. In this study, characteristic SOA tracers of these precursors were quantified in ambient samples of PM2.5 taken in Hong Kong and their contributions to SOA were estimated using a tracer-based method. Samples were collected every other day from four sampling sites during a field measurement campaign in the summer of 2006. Fourteen SOA tracers, along with 24 other polar oxygenated compounds, were identified and quantified using gas chromatography/ion trap mass spectrometry with prior trimethylsilylation. Concentrations of the individual tracers ranged from a few tenths to a few hundreds ng m−3. The tracer concentrations were found to be 1 order of magnitude higher on days under regional transport influences due to elevated oxidant levels than on days under mainly local emissions influences. Using the measured SOA tracer concentrations in the ambient aerosols and laboratory-derived tracer mass fractions reported by Kleindienst et al. (2007), we estimated that the average SOA attributable to isoprene, monoterpenes, β-caryophyllene, and toluene was 8.83 μg m−3 on days under regional transport influences versus 0.99 μg m−3 on days under mainly local emissions influences, accounting for approximately 49% and 21%, respectively, of the ambient OC concentrations. The tracer-based estimates indicate that monoterpenes and β-caryophyllene are significant contributors to ambient PM2.5 in the summer, which may be due to the high emissions of these biogenic hydrocarbons in Hong Kong.