AbstractMolecular I₂ can be produced from iodide‐based lead perovskites under thermal stress; triiodide, I₃⁻, is formed from this I₂ and I⁻. Triiodide attacks protic cation MA⁺‐ or FA⁺‐based lead halide perovskites (MA⁺, methylammonium; FA⁺, formamidinium) as explicated through solution‐based nuclear magnetic resonance (NMR) studies: triiodide has strong hydrogen‐bonding affinity for MA⁺ or FA⁺, which leads to their deprotonation and perovskite decomposition. Triiodide is a catalyst for this decomposition that can be obviated through perovskite surface treatment with thiol reducing agents. In contrast to methods using thiol incorporation into perovskite precursor solutions, no penetration of the thiol into the bulk perovskite is observed, yet its surface application stabilizes the perovskite against triiodide‐mediated thermal stress. Thiol applied to the interface between FAPbI₃ and Spiro‐OMeTAD (“Spiro”) prevents oxidized iodine species penetration into Spiro and thus preserves its hole‐transport efficacy. Surface‐applied thiol affects the perovskite work function; it ameliorates hole injection into the Spiro overlayer, thus improving device performance. It helps to increase interfacial adhesion (“wetting”): fewer voids are observed at the Spiro/perovskite interface if thiols are applied. Perovskite solar cells (PSCs) incorporating interfacial thiol treatment maintain over 80% of their initial power conversion efficiency (PCE) after 300 h of 85 °C thermal stress.