Acetate is likely remnant from PbI2 synthesis which commonly involves mixing water soluble Pb(OAc)2 or Pb(NO3)2 salts with KI to precipitate PbI2. Spiking experiments quantified the OAc concentration as ~2.2 ± 0.3 mol% with respect to Pb (Supplementary Figure ). It is particularly interesting to note the agreement between our inferred synthetic origin of SA2 and reports by Eckstein et al. that the Pb(OAc)2 route to PbI2 typically results in lower stoichiometry than via Pb(NO3)2. The measured amount of OAc accounts for only one fourth of the full 8% iodide deficiency in SA2, implying other extrinsic impurities are present in similar percentages.