Jar testing with aluminum sulfate was performed on 31 real waters from a variety of sources across the US to comprehensively evaluate the criteria for determining compliance with enhanced coagulation as specified in the proposed D/DBP Rule. The majority of the utilities that supplied water for this study are expected to have difficulty in meeting the proposed total organic carbon (TOC) removal requirements and may decide to achieve compliance with enhanced coagulation through one of the alternative performance criteria. Waters that contained low TOC concentrations were least likely to meet the proposed TOC removal requirements at alum doses lower than was needed to achieve one of the alternative performance criteria. Those waters that contained high TOC concentrations and low alkalinities were most likely to meet the proposed TOC removal requirements. The jar-test results provided insight into the effects of pH and humic content on the coagulation of natural organic material (NOM). The alum doses necessary for the coagulation of NOM increase with increases in initial alkalinity, and corresponding increases in coagulation pH. A stoichiometry was observed between the alum dose needed to treat a water for turbidity and organics removal and a water's initial TOC concentration when coagulation occurred at pH values between 5 and 6. Coagulation of NOM was most effective for waters high in humic content, designated by large hydrophobic contents and specific ultraviolet absorbances. Alum was observed to react preferentially with certain hydrophobic organic substances. Many hydrophilic organic substances were removed only after the coagulant demand for the reactive hydrophobic organics had been satisfied.