The ion/molecule reactions of H3O+ ions with a series of sesquiterpenes have been studied in view of their detection by CIMS. For all sesquiterpenes multiple product ions have been observed, but the protonated molecular parent ion is always the major ion species with yields ranging from 30 to 94 %. All reactions proceed at the collision rate, which is calculated to be close to 3×10-9 cm3molecule-1 s-1. Preliminary studies indicate that NO+ may also be a potential reagent ion for sesquiterpene detection.