This study examined the trihalomethane formation potential (THMFP) from water extracts of organic fractions of a mineral soil (Scriber Clay Loam) and an organic soil (Rindge Muck) in the Sacramento- San Joaquin Delta. Organic carbon from the Delta soils, especially peat soils, is of concern because they have been identified as important sources of trihalomethane (THM) precursors in the Delta drinking water sources. In addition, the effects of Na+ and Ca2+ on the partitioning of organic carbon and THM precursors in these soils were evaluated. Water extracts (1:100, soil : water) were sequentially filtered through a series of pore size filters: 1.2, 0.45, 0.1 and 0.025 im. Each filtrate was analyzed for total organic carbon, UV absorbance at 254 nm (UV254), and THMFP. The authors defined organic matter with size fractions of 1.2-0.45 im, 0.45-0.1 im, 0.1-0.025 im and smaller than 0.025 im as particulate (POC), colloidal (COC), fine colloidal (FCOC) and dissolved organic carbons (DOC), respectively. Results showed that the dominant fraction in all extracts was DOC, followed in a decreasing order by COC, POC and FCOC. The DOC from the organic soil was 85%, whereas it was 57% from the mineral soil. Filtrates of water extracts through 0.45 im from the mineral soil contained 36% COC. Such a high content of colloidal materials in water caused high turbidity and resulted in abnormally high UV254 values. Monovalent sodium had little impact on the distribution of each fraction, but divalent calcium significantly reduced COC. The difference in THMFP among size fractionations was relatively small. Thus, the authors believe that DOC contains the majority of THM precursors. Ideally, DOC, organic fractions that passed through 0.025 im, should be used in characterizing THM precursors in water; however, slow filtration makes it impractical for large volumes. The difference in total organic carbon of 0.1 im and 0.025 im filtrates was only about 2%. Thus, a 0.1 im pore-size filter is recommended for THM characterization. Includes 16 references, tables, figures.