Separation of Brimfield and Brookfield soil series from their competing series based primarily on parent material and color hue has proven difficult. The objective of this study is to examine the value of extractable iron in separating Brimfield (Loamy, mixed, mesic active Lithic Dystrudepts) and Brookfield (Coarse-loamy, mixed, mesic, active Typic Dystrudepts) soil series from their competing series (Lithic and Typic Dystrudepts). A second objective is to test the hypothesis that these soils may actually be fossil acid sulfate soils (FAS). A model was developed with Arc View v3.3 to identify currently mapped Brimfield and Brookfield soils and probable areas of mismapped competing series in Connecticut and Massachusetts. Forty pedons were sampled and characterized through the National Cooperative Soil Survey Program. Initial results indicate significant differences in mean dithionite-citrate extractable iron (Fed) of 2.50% FAS and 1.01% in competing series, with 2.26% in mis-mapped competing series (probable FAS). Ammonium oxalate extractable iron ratios of (Feo) to Fed were 0.11 for FAS, and 0.15 for probable FAS, which differed significantly from the 0.22 ratio for the competing series. Additionally, mean pH (CaCl2) in FAS (4.1) and probable FAS (4.1) differ significantly from the competing series (4.4). Mean Fed, Feo:Fed, and pH (CaCl2) support the hypothesis that Brimfield and Brookfield series are distinctly different from their competing series, are FAS, and may be separated from competing series by extractable iron. This work is supported by USDA Natural Resources Conservation Service (NRCS).