Plants emit significant amounts of monoterpenes into the earth’s atmosphere, where they react rapidly to form a multitude of gas phase species and particles. Although BVOCs emissions are strongly modulated by the biotic and abiotic environment, studies showed that monoterpenes emissions can be genetically fixed, highlighting the possible existence of distinct chemotypes within a tree species [1], [2]. Within the activity of the COV3ER project (Biogenic volatile organic compounds (BVOC) emissions by managed ecosystems: new references over French crops and forest and management effects), the emission of biogenic volatile organic compounds, especially monoterpenes, from Quercus ilex was studied in the forest of Puechabon, south of France. Measurements were carried out from mid of June to mid of July, 2018using a fastGC/PTR-MS system connected to dynamic branch enclosures. The deployed fastGC is an upgraded version of the one adopted by Materic et al., 2015, added to a PTR-MS and tested in order to optimize the separation of monoterpenes depending on their volatility. Investigations focused mainly on characterizing the diurnal variability of monoterpenes chemical nature and concentration, with a fine time resolution from different sources on this natural site. Data analysis revealed the presence of three different genetically fixed emission patterns, known as chemotypes in this forest ecosystem. As reported in previous studies, Quercus ilex emissions increased with light. However, the chemical speciation as well as the rate of monoterpenes emissions dependency on temperature and light still need to be investigated.