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Lead halide perovskites are generally formed from a reaction of the lead halide salt with the halide salt of the A cation. The effects of varying film compositions as result of nonstoichiometric precursor ratios on electronic properties of halide perovskites are currently under debate. It is imperative, but experimentally challenging, to determine the chemical composition of thin films as a function of precursor ratio for a full understanding of the effect. Herein we report a precise quantification of the methylammonium (MA) content in differently fabricated films of MAPbI3 via infrared spectroscopy. We compare the thin film data to the first high-quality dielectric function obtained from single crystals with IR ellipsometry. For spin-coated samples, we find that the MAI/PbI2 ratio in solution has an effect on the MA content in the resulting thin films, which is in the range of 77–106 % compared to single crystals. For coevaporated samples, we show that a very similar range of MA content variation is accessible. Interestingly, coevaporated films prepared like those for high-efficiency solar cells have a significantly lower MA content than that of single crystals.