The instability of perovskite solar cells is one of the major bottlenecks to their industrialization, and understanding exact degradation mechanisms at the atomic-level in perovskite absorber layers is a crucial step in improving long-term stability. Among environmental factors, light-induced degradation obviously plays a particularly important role for solar cells. Here, we determine the degradation products and kinetics of scraped thin-film samples of methylammonium lead iodide (MAPbI3) and formamidinium methylammonium lead iodide (FA0.95MA0.05PbI3) compositions in real time using in situ light irradiation in magic-angle-spinning solid-state NMR spectroscopy. Our findings show that after 130 hours of intense illumination, the MAPbI3 absorber material completely degrades into PbI2 at a rate of 64 nmol/h. Furthermore, no formation of Pb(0) of was observed. On the other hand, in a similar time, the FA0.95MA0.05PbI3 perovskite degraded by only approximately 50%, at a rate of 30 nmol/h, clearly confirming that the formamidinium containing perovskite is more stable than the pure MAPbI3. The in situ approach introduced here facilitates the direct observation and comparison of the stabilities of different perovskite compositions over short periods of time.