The reactions of monoterpenes and related C 1 0 compounds with H 3 O + in a proton transfer reaction-mass spectrometer (PTR-MS) were studied, with a view to better understanding the signal produced by this instrument when detecting these compounds. The monoterpenes α- and β-pinene, 3-carene and limonene produced fragment ions of masses 67, 81 and 95 as well as a protonated molecular ion of mass 137, while p-cymene (C 1 0 H 1 4 ) produced ions of masses 41, 91, 93 and 119 in addition to mass 135. The fragmentation patterns were observed to vary as E/N was varied. Camphor (C 1 0 H 1 6 O) did not fragment within the E/N range 80-120Td. The proton transfer reaction rate coefficients for these monoterpene species with H 3 O + were found to be 2.2x10 - 9 to 2.5x10 - 9 cm 3 s - 1 . For camphor the rate coefficient was 4.4x10 - 9 cm 3 s - 1 . Water vapour pressure in the inlet air affected the fragmentation pattern for p-cymene, limonene and 3-carene. The uncertainties associated with the PTR-MS measurement of these compounds are discussed.