The primary oxidation steps of (γ-terpinene+OH) and (d-limonene+OH) reactions are investigated using two techniques: an excimer laser photolysis set-up coupled with UV absorption spectrometry performed at atmospheric pressure and a fast-flow reactor coupled to time of flight mass spectrometry at low pressure. OH radicals are generated either by photolysis of H(2)O(2) or via the reaction of H atoms with NO(2). The primary reaction of monoterpenes with hydroxyl radicals can proceed by two reaction pathways: OH-addition and H-abstraction. The branching ratios for these channels are measured at various pressures for (γ-terpinene+OH) and (d-limonene+OH) reactions and a discussion on the H-abstraction importance for reactions of monoterpenes with hydroxyl radicals is proposed. H-abstraction may contribute to (31±9)% and (34±8)% respectively, for γ-terpinene and d-limonene reactions with OH at atmospheric pressure and respectively to (28±6)% and (28±8)% at low pressure (between 0.5 and 2.8 torr). As already pointed out by the Leuven group of Peeters, H-abstraction may be a significant reaction pathway for the reactions of monoterpenes with hydroxyl radicals. Therefore, oxidation products resulting from the H-abstraction should not be neglected in the mechanisms describing the formation of secondary organic aerosols (SOA) from gas-phase reactions of monoterpenes+OH.