Accelerating forest fire phenomenon is studied in this paper. This phenomenon characterizes fires with a sudden increase of the rate of spread and of the energy released without any changes in the meteorological and topographic conditions. The thermochemical explanation is investigated in this study: acceleration of the rate of spread could be the consequence of the ignition of Biogenic Volatile Organic Compounds emitted by fire heated vegetation. Two experimental setups are used to perform very simple experiments in order to give validation elements to this thermochemical approach. The first experimental setup used permits to show that gases emitted by vegetation (even if they are hot) are diffused mainly in the direction of the ground. The second one shows that the gases emitted by heated Rosmarinus officinalis needles can be ignited with a pilot flame. Rosmarinus officinalis is a typical vegetal species involved in accelerating forest fires. This work does not prove neither that this hypothesis has been validated nor that this is the only explanation for accelerating forest fire accidents, it only brings arguments for future discussions and for a possible validation of the thermochemical approach.