Vapor phase deposition of formamidinium-rich perovskites is hampered by the decomposition of formamidine in the vapor phase. We use the reversibility of this reaction to establish a dynamic equilibrium that enables the vapor-phase synthesis of formamidinium iodide from the “degradation products” s-triazine, ammonia, and hydrogen iodide and thereby convert lead halide thin films to lead halide perovskites. Finally, we produce the first proof of concept solar cells via this innovative process.