Product ion distributions (PIDs) for the reactions of seven sesquiterpenes (aromadendrene, β-caryophyllene, α-cedrene, α-humulene, isolongifolene, longifolene and δ-neoclovene) with H3O+, NO+ and O2{radical dot}+ reagent ions have been determined in a selected ion flow tube (SIFT) instrument operated at 1.4 mbar and 297 K. The H3O+/sesquiterpene reactions mainly proceed by non-dissociative proton transfer (C15H25 +, m/z 205), except for the reactions of H3O+ with β-caryophyllene and α-humulene which are characterized by more pronounced fragmentation. Two groups of fragment ions of which the components are separated by CH2 units, are generally observed. The NO+/sesquiterpene reactions mainly result in the charge transfer product (C15H24 +, m/z 204). The NO+/sesquiterpene charge transfer yield is generally larger than the protonated sesquiterpene yield for the corresponding H3O+/sesquiterpene reactions. A small amount of association product (C15H24·NO+, m/z 234) is found for the sesquiterpenes aromadendrene, β-caryophyllene, longifolene and δ-neoclovene. O2{radical dot}+/sesquiterpene reactions are found to proceed mainly by dissociative charge transfer, resulting in severe fragmentation. The collisional rate constants for these reactions were calculated using the parameterized theory of Su and Chesnavich, taking into account polarizabilities and electrical dipole moments of the sesquiterpenes as derived from B3LYP/aug-cc-pVDZ quantum chemical calculations. Experimental rate constants for the reactions of NO+ and O2{radical dot}+ with the sesquiterpenes were obtained relatively to the calculated H3O+/sesquiterpene rate constants. Absolute H3O+/sesquiterpene rate constants were obtained for β-caryophyllene, α-cedrene, α-humulene and longifolene. Large but systematic discrepancies between experimentally determined rate constants and calculated collision rate constants suggest the need for a re-evaluation of sesquiterpene saturation vapor pressure values. In view of possible detection of sesquiterpenes in moist samples, the influence of water vapor on the ion chemistry was investigated and the reaction rate constants of H3O+·H2O and NO+·H2O with sesquiterpenes were obtained relative to those of bare H3O+ and NO+, respectively.