Thermal decomposition of high-fluorine content PFAS streams for the disposal of old generations of concentrates of firefighting foams, exhausted ion-exchanged resins and granular activated carbon, constitutes the preferred method for destruction of these materials. This contribution studies the thermal transformation of perfluoropentanoic acid (C4F9C(O)OH, PFPA), as a model PFAS species, in gas-phase reactions over broad ranges of temperature and residence time, which characterise incinerators and cement kilns. Our focus is only on gas-phase reactions, to formulate a gas-phase submodel that, in future, could be used in comprehensive simulation of thermal destruction of PFAS; such comprehensive models will need to comprise fluorine mineralisation on flyash and in clinker material. Our submodel consists of 56 reactions and 45 species, and includes new pathways that cover the initial decomposition channels of PFPA, including those that lead to the formation of the n-C4F9 radical, the abstraction of hydroxyl H by O/H radicals, the fragmentation of the n-C4F9 radical, reactions between HF and perfluoropentanoic acid, as well as between HF and heptafluorobutanoyl fluoride (C3F7COF), and the cyclisation reactions. The model illustrates the formation of a wide spectrum of small CnFm and CnHFm compounds in the temperature window of 800–1500 K, 2 and 25 s residence time in a plug flow reactor, providing theoretical estimates for the operating conditions of PFAS thermal destruction systems. The initiation reactions involve the loss of HF and formation of the transition α-lactone species that converts to C3F7COF, with C4F9C(O)OH completely decomposed at 1020 K for 2 s residence time. At 1500 K, we predict the emission of ꞉CF2 (biradical difluorocarbene), HF, CO2, CO, CF4, C2F6, and C2F4, but at < 1400 K, we note the formation of 1H-nonafluorobutane (C4HF9), phosgene (COF2), and heptafluorobutanoyl fluoride (C3F7COF), with 1-C4F8, 2-C4F8 and C3HF7 persisting to 1500 K. We demonstrate that, the gas-phase pyrolysis processes by themselves convert PFAS to HF and short-chain fluorocarbons, with similar product distribution for short (2 s) and long (25 s) residence times, as long as the treatment temperature exceeds 1500 K. These residence times reflect those encountered in incinerators and cement kilns, respectively. Thermokinetic and mechanistic insights revealed herein shall assist to innovate PFAS thermal disposal technologies, and, from a fundamental perspective, to accelerate research progress in modelling of gas/solid reactions that mineralise PFAS-derived fluorine.