Natural organic matter (NOM) is capable of interfering with Fe hydrolysis and influencing the size, morphology, and identity of Fe precipitates. Conversely, Ca2+ raises surface potential and increases the size and aggregation of Fe precipitates, leading to more effective coagulation and widening the pH range of water treatment. Experiments and modeling were conducted to investigate the significance of the Fe/NOM ratio and the presence of Ca2+ in coagulation. At the high Fe/NOM ratio, sufficient or excess Fe was available for NOM removal, and coagulation proceeded according to expectations based upon the literature. At the low Fe/NOM ratio, however, NOM inhibited Fe hydrolysis, reduced zeta potential, and suppressed the formation of filterable Fe flocs, thereby interfering with NOM removal. In these dose-limited systems without Ca2+, complexation of Fe species by NOM appears to be the mechanism by which coagulation is disrupted. Equilibrating NOM with 1 mM Ca2+ in dose-limited systems prior to dosing with FeCl3 increased Fe hydrolysis and zeta potential, decreased the fraction of colloidal Fe, and improved NOM removal. In systems with Ca2+, data and modeling indicate that Ca2+ complexation by NOM neutralizes some of the negative organic charge and minimizes Fe complexation, making Fe species available for hydrolysis and effective coagulation. This finding represents an important advance in understanding not only how Ca2+ may improve coagulation outcomes, but also in predicting the conditions under which Ca2+ may prove beneficial.