An exhaustive assessment of the behaviour of virgin and mechanically reprocessed poly(ethylene terephthalate) (PET) facing thermal and thermo-oxidative decomposition processes is presented in this work, as an approach for the energetic valorisation of post-consumer PET goods. Multi-rate linear-non-isothermal thermogravimetric (TGA) experiments under inert (Ar) and reactive (O2) conditions were performed to virgin PET and its recyclates in order to simulate the thermal behaviour of the materials facing pyrolysis and combustion processes. The release of gases was monitored by evolved gas analysis of the fumes of the TGA experiment, by in-line Fourier-transform infrared (IR) analysis, with the aid of 2D-correlation IR characterisation. A kinetic analysis methodology, consisting in the combination of six different methods (namely Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, Vyazovkin, Master-Curves and Perez-Maqueda criterion along with Coats-Redfern equation) was applied. Its validity for being used for both constant and variable kinetic parameters was discussed. The kinetic model that described both thermal and thermo-oxidative decompositions of PET and its recyclates was of the type An: nucleation and growth of gas bubbles in the melt. Novel parameters and functions were proposed to characterise the thermal stability along the reprocessing cycles, as well as the variation of the activation energy and the pre-exponential factor during thermal and thermo-oxidative decompositions. The reliability of a simplified kinetic triplet with constant activation parameters was suitable only under thermal decomposition. The usability of PET after reprocessing showed a threshold in the thermal performance from the second recyclate on. During thermal and thermo-oxidative processes, reprocessed PET behaved similarly to virgin PET, and thus current energetic valorisation technologies could be assimilable for all materials.