In 1990, the water utility of Hopkinton, Massachusetts, began adding a commercial polyphosphate product to two of its five wells to address red-water complaints. Testing under the Lead and Copper Rule revealed that the utility was in violation for lead: 300 ug Pb/L maximum and 77 ug Pb/L at the 90th percentile. Two section of lead service line were supplied by the Hopkinton utility to the US Environmental Protection Agency for complete mineralogical and elemental characterization. Various sampling techniques, analytical determinations and computer modeling approaches were used with background water chemistry data to elucidate the causes of the high plumbosolvency. It was established that the polyphosphate added to sequester iron and manganese caused a change in the thermodynamic stability of the mineral cerussite (PbCO3) making it unstable. The mineral pyromorphite (Pb5(PO4)3Cl) became the stable phase, but was precipitated as needle-like laths that failed to isolate the underlying cerussite from the water. Includes 13 references, tables, figures.