The reactions of monoterpenes and related C10 compounds with H3O+ 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 (C10H14) 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 (C10H16O) did not fragment within the E/N range 80–120 Td. The proton transfer reaction rate coefficients for these monoterpene species with H3O+ were found to be 2.2×10−9 to 2.5×10−9 cm3 s−1. For camphor the rate coefficient was 4.4×10−9 cm3 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.