Over the past 10 years, organometal halide perovskites have revolutionized the field of optoelectronics, particularly of emerging photovoltaic technologies. Today’s best perovskite solar cells use triple-cation compositions containing a mixture of formamidinium, methylammonium, and cesium to enable more reproducible and stable device performance. The common procedure uses as-prepared precursor ink to avoid an undesirable decrease in device performance, attributed recently to a chemical reaction between methylammonium and formamidinium in solution. Here we employ nuclear magnetic resonance spectroscopy to explore different experimental conditions that can significantly modify these reaction kinetics; in particular, we find that the presence of cesium as the third cation can substantially slow down methylammonium-formamidinium reactivity. Our findings allow us to draw up a protocol for extended overtime perovskite ink stabilization. Insight into the reactivity of perovskite inks under different experimental conditions Addition of methylamine to formamidinium gives trans / cis N -methylformamidinium Cs + significantly stabilizes the precursor solution by slowing down the reaction NMR is a powerful tool to predict solution modifications prior to device fabrication Halide perovskite inks are unstable over time, which negatively effects photovoltaic device performance. This is due to a reaction between the species in solution, studied here using NMR by Valenzano et al., and the kinetics of this can be tuned by adjusting the components/experimental conditions.