The ability to measure sesquiterpenes (SQT; C15H24) by a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) was investigated. SQT calibration standards were prepared by a capillary diffusion method and the PTR-MS-estimated mixing ratios were derived from the counts of product ions and proton transfer reaction constants. These values were compared with mixing ratios determined by a calibrated Gas Chromatograph (GC) coupled to a Flame Ionization Detector (GC-FID). Product ion distributions from soft-ionization occurring in a selected ion drift tube via proton transfer were measured as a function of collision energies. Results after the consideration of the mass discrimination of the PTR-MS system suggest that quantitative SQT measurements within 20% accuracy can be achieved with PTR-MS if two major product ions (m/z 149+ and 205+), out of seven major product ions (m/z 81+, 95+, 109+, 123+, 135+, 149+ and 205+), are accounted for. Considerable fragmentation of bicyclic sesquiterpenes, i.e. β-caryophyllene and α-humulene, cause the accuracy to be reduced to 50% if only the parent ion (m/z 205+) is considered. These findings were applied to a field dataset collected above a deciduous forest at the PROPHET (Program for Research on Oxidants: Photochemistry, Emissions, and Transport) research station in 2005. Inferred average daytime ecosystem scale mixing ratios (fluxes) of isoprene, sum of monoterpenes (MT), and sum of SQT exhibited values of 15 μg m-3 (4.5 mg m-2 h-1), 1.2 μg m-3 (0.21 mg m-2 h-1), and 0.0016 μg m-3 (0.10 mg m-2 h-1), respectively. A range of MT and SQT reactivities with respect to the OH radical was calculated and compared to an earlier study inferring significantly underestimated OH reactivities due to unknown terpenes above this deciduous forest. The results indicate that incorporating these MT and SQT results can resolve  30% of missing OH reactivity reported for this site.