A comprehensive description of halide perovskite degradation is still lacking. Light and temperature are herein combined as stressors to gain novel insights into the involvement of free carriers in the degradation of (FA_0.73MA_0.27)Pb(I_0.945Br_0.055)₃ and spiro-OMeTAD. In situ EPR spectroscopy is at the core of this study because of its ability to probe free carriers with high sensitivity. The results are corroborated with in situ X-ray diffraction, and thermogravimetric and calorimetric analysis to link the generation of free carriers with long-range structural modification, gas release and heat exchange during degradation. It is highlighted that temperature-induced perovskite decomposition does not involve radicals, in contrast to the final stage of the decomposition, which involves radicals localized on the formamidinium. When combined with light, the rise in spin concentration correlates with the increasing rate of the degradation compared to that in darkness. The de-doping reaction of spiro-OMeTAD is observed up to its crystallisation temperature (128 °C). Finally, by combining light, temperature and an external magnetic field, we provide the first evaluation of the room-temperature exciton binding energy for (FA_0.73MA_0.27)Pb(I_0.945Br_0.055)₃, for which a value of 43 meV was determined.