EPR is particularly adept at detecting unpaired electrons, making it a powerful tool for studying radicals and their role in material decomposition. Moreover, it allows for direct observation of radical formation and dynamics. This EPR technique has been widely used to investigate the decomposition mechanisms of polymers and biological compounds. However, despite its utility, this approach has not been used to study the decomposition of HPs. Notably, reactive oxygen species like OH • and O2 •-radicals have extremely short half-lives of 10⁻⁹ and 10⁻⁶ seconds, respectively. Spin-trapping EPR enables the detection of short-lived radicals by forming stable adducts. Spin traps like 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) enable their detection at room temperature, with distinctive EPR spectra measurable for up to 35 min, and have been effectively used to study radical generation in X-ray-irradiated gold nanoparticles. BMPO has been used effectively to study radical generation by gold nanoparticles irradiated with X-rays. Here, we show a unique way to study the decomposition process of HP-NCs. We are monitoring the ability of the 3D CsPbBr3 and 0D Cs4PbBr6 NCs to generate radicals as a response to visible light illumination in a polar solvent. The generated electrons are migrating and confined by the BMPO spin trap. Using the EPR technique, we can spot the moment when the NCs are starting to decompose. Moreover, we follow this process, varying the amount of a polar solvent and the duration of the illumination. The distinctive EPR spectra provide information on the stability of the studied HPs-NCs.