The pursuit of highly efficient and cost-effective photovoltaic materials has led to the emergence of organic-inorganic metal halide perovskites. However, their inherent instability poses challenges to their practical application, limiting their lifespan and scalability. Mixed tin-lead compositions have garnered significant attention due to their unique optoelectronic features and small bandgaps, offering promise for various applications. Nonetheless, their low ambient stability presents a significant hurdle that requires a thorough investigation into their degradation mechanisms. This study aims to understand the degradation mechanisms of tin-lead perovskites, with a specific focus on the role of halide chemistry and the impact of iodine on their stability. Our findings reveal a cyclic degradation process, where iodine and SnI4 act as key degradation products, compromising the stability of the perovskite material. Furthermore, the presence of triiodide, derived from native iodine oxidants, exhibits a strong correlation with degradation. We observe that the selection of A-site cations significantly influences the oxidation stability of Sn-Pb perovskites. Cesium-rich phases and solar cells demonstrate superior resistance to oxidative stress compared to their methylammonium-based counterparts, primarily due to the limited formation of triiodide. Leveraging this insight, we successfully stabilize sensitive methylammonium-based Sn-Pb perovskite films and devices against oxidation by employing CsI coatings. This practical approach provides essential guidelines for enhancing the stability of perovskite materials and devices. The significance of this study lies in its contribution to the design and engineering of perovskite materials and devices. Understanding the role of iodine in perovskite deterioration is crucial for improving their stability and durability, thereby paving the way for their commercialization. By elucidating the degradation mechanisms of tin-lead perovskites, we can develop effective strategies to mitigate their degradation, enhance their stability and lifespan, and unlock their full potential for various photovoltaic applications. This work aligns with the objectives of the scientific program it is part of, as it addresses the challenges associated with the stability of perovskite solar cells. Stable and efficient perovskite solar cells play a vital role in renewable energy production, contributing to a more sustainable and environmentally conscious future. By overcoming the degradation issues and enhancing the stability of tin-lead perovskite materials, this research contributes to the development of advanced photovoltaic materials.