A series of filtration steps was used to separate iron (Fe) and manganese (Mn) into soluble, colloidal, and particulate fractions. Two case studies demonstrate how fractionation data can improve understanding of the origins and removal of Fe and Mn. The Mn species present in one water source were shown to be regulated by a biogeochemical cycle in which the natural oxidation and reduction of Mn appeared to be microbially mediated. Mn removal was improved by adjusting the application of potassium permanganate to account for this cycle. A groundwater treatment plant was suffering from high postfilter Fe and Mn concentrations. Fractionation data identified the problem as inadequate solids capture, not oxidant dosage, allowing a quick solution. The unexpected oxidation of Mn with chlorine (Cl2) was attributed to Fe oxide surface catalysis. Without adequate particle removal (no coagulant added), applying Cl2 on top of the filters exhibited superior removal to adding Cl2 before a detention basin. Includes 15 references, tables, figures.