and lower device performance (Fig. ) after recrystallization. The stoichiometry and device results are only partially explained by OAc and KI impurities, suggesting perovskite material quality is sensitive to other trace impurities that remain difficult to identify. For example, increased nitrogen oxide SIMS intensities for AA4 (Fig. ) suggest elevated nitrate impurities in this reagent that was removed by purification (Supplementary Figure ). Indeed, several additional molecular impurities that might impact perovskite synthesis are visible by 1 H-NMR of the as-received PbI2s (Fig. ). There are unassigned, hydrogen-containing impurities present in all as-received PbI2 sources except AA5 (SA2 additionally showed a small impurity peak downfield at 10.7 ppm). Recrystallized materials in Fig. display reduced concentrations of original 1 H-NMR impurities (compare, for example, the quartet at 3.54 ppm in AA4 and AA4-RC in Fig. and Fig. , respectively), yet contain small signals from new impurities, one of which was identified as isopropanol (IPA, doublet at 0.99 ppm) that we used during filtration and collection. The NMR and additional SIMS data (Supplementary Fig. ) all indicate the recrystallization decreases impurity concentrations in most categories. However, OAc and K removal by recrystallization exemplify a competition between detrimental and beneficial impacts of impurities. Therefore, while recrystallizing PbI2 may increase the purity, the degree to which the purified reagent improves the perovskite active layer and resulting devices may vary due to a given process's sensitivity to the trace beneficial impurities removed, as well as new trace species introduced, by the purification. Consequently, researchers may leverage this information about the different major impurities in as-received PbI2 reagents or incorporate a purification protocol to best suit their goals. Our observations help rationalize difficulties reproducing results with PbI2 from different suppliers or even batch-to-batch reproducibility from a single supplier. Improvements to purification methods and analytical characterization are necessary efforts for fundamental halide perovskite studies; accurate quantification of the impact of impurities and additives requires first reducing their levels to negligible concentrations before titrating back into the reagents. Ultimately, aqueous recrystallization methods may not achieve a desirable purity level nor optimal perovskite quality in devices, but alternative methods such as dissolution and precipitation from dimethylsulfoxide (DMSO) or sublimation in the dark (to avoid photolysis ) under vacuum remain viable and should be simultaneously explored. Although the ideal purification technique relevant to perovskite semiconductors remains to be identified, fundamental-and device-oriented research will benefit from deeper analytical insight into perovskite precursors and the resulting materials.