Isaac Newton spent some four decades researching “chymistry,” the early modern equivalent of our chemistry. Although his laboratory notebooks survive, his experimental goals remain obscure to the present day. Our work reveals that Newton was engaged in fruitful chemical research even by modern standards. Replication of his experiments, involving Newton’s “vitriol” (from his “liquor of antimony,” NH4Cl, HNO3, and Sb2S3) and verdigris (Cu(CH3COO)2), produced a variety of NH4+-, Cl−-, SO4−2-, NO3−-, and Cu-containing crystallization products. We analyzed these products using powder X-ray diffraction (XRD) (Cu Kα radiation) and Rietveld refinement, which revealed a complex mixture of (NH4)2Cu(SO4)2(H2O)6, NH4NO3, NH4Cl, (NH4)2CuCl4(H2O)2, and (NH4)[Cu(NH3)2Cl3]⋅2H2O. The XRD data also consistently showed a suite of peaks unmatched by any phase in the PDF-5 database. A crystal of the unknown product was analyzed using single-crystal X-ray methods (Mo Kα radiation), revealing a previously unknown compound, (NH4)2[Cu2Cl2(C2H3O2)4]·2NH4Cl, with space group Pmna and room-temperature unit-cell parameters of a = 14.550(3) Å, b = 8.850(1) Å, and c = 9.116(2) Å. The inclusion of this phase in the Rietveld refinements yielded a satisfactory fit. Our ongoing replications of Newton’s crystallization experiments reveal that his research produced a complex, unusual suite of phases, including the aforementioned previously unknown compound.