Homes with lead service lines (LSLs) in the City of Brandon, Manitoba, Canada, were found to exceed the provincial standard of 10 μg/L for lead in drinking water. Solids identified by X-ray diffraction of LSL scale were Pb5O8 and PbO2, indicating that lead(II) solids in the LSL scale have been oxidized to lead(IV) solids by free chlorine residuals. Natural organic matter (NOM) can reduce PbO2 within a few hours, and Brandon treated water has high levels of NOM at approximately 5–7.6 mg/L as total organic carbon (TOC). As water stagnates in the LSL during periods of no water use the free chlorine residual is depleted, permitting PbO2 to oxidize NOM and be reduced to more soluble lead(II) species, resulting in an increase in dissolved lead concentrations. Although it is generally believed that aquatic humic substances (AHS) are primarily responsible for the reductant capacity of NOM, removal of AHS from the treated water resulted in a 6% decrease in lead release from PbO2, while removal of 50% of total NOM resulted in a 75% decrease in lead release. AHS and TOC were not found to play a significant role in the reduction of PbO2 in this water.