The ion/molecule reactions of H3O+, NO+, and O2 + with six monoterpenes (α-pinene, β-pinene, limonene, Δ3-carene, myrcene, and camphene) have been studied at 298K and 1.47mbar using a selected ion flow tube (SIFT). The rate constants and the ionic products of the reactions have been determined. It turns out that all reactions occur at the collisional rate. Quantum chemical calculations were performed of the polarizability and dipole moments of all six monoterpenes studied, to determine the collisional rate constant using the Su-Chesnavich approach. Our results show that the H3O + reactions with the monoterpene M proceed via proton transfer and result in the protonated molecules MH+ (m/z=137) and partly in an ion with m/z=81. The NO+ reactions proceed via charge transfer resulting mainly in the ionized monoterpene C10H16 +. The reactions with O2 + proceed via dissociative charge transfer giving rise to several ionic products, which have also been observed in electron impact spectra of these monoterpenes.