We have studied the reactions of H 3O +, NO + and O 2 + with five oxidation products of monoterpenes: pinonaldehyde (oxidation product of α-pinene), nopinone (β-pinene), α-pinene oxide (α-pinene), 4-acetyl-1-methylcyclohexene (d-limonene) and caronaldehyde (Δ 3-carene). It was found that all reactions studied proceed with a rate constant close to the collision rate constant, calculated with the parameterized equation of Su and Chesnavich. The dipole moment and polarizability of the neutral reactants, needed for the calculation of these collision rate constants, were determined by quantum chemical calculations. Analysis of the ion product spectra shows that H 3O + reacts with 4-acetyl-1-methylcyclohexene and nopinone by non-dissociative proton transfer. The major channel of the reaction of H 3O + with α-pinene oxide, pinonaldehyde and caronaldehyde is elimination of a water molecule following protonation. The reaction of NO + with 4-acetyl-1-methylcyclohexene and nopinone is mainly determined by charge transfer. Charge transfer is also observed in the reaction of NO + with α-pinene oxide, pinonaldehyde and caronaldehyde, as well as a series of fragment ions. A non-negligible hydride ion transfer channel also occurs in the product spectra of NO + with pinonaldehyde. For the NO +-nopinone and NO +-caronaldehyde reaction also three-body association is observed. The product spectra of O 2 + with the neutral reactants show multiple products, mainly characterized by charge transfer and fragmentation, and are less suited to be used for CIMS detection of the monoterpene oxidation products.