Concentrations of dissolved selenium (Se) as large as 330 µg 1-1 have been detected in water samples from backfill aquifers in the southern Powder River Basin (PRB) of northeastern Wyoming. During surface coal mining, rock material overlying the coal is redistributed in relation to its original stratigraphic position; exposure of the previously buried material to surface oxidizing conditions decreases the stability of sulfides and organic matter containing Se, increasing Se concentrations in associated ground waters. Groundwater and backfill-core samples were collected from three sites located at two active mines in the PRB. Dissolved organic carbon (DOC) in the groundwater samples was domimrtoo by the acid fraction (sum of hydrophobic and hydrophilic acids) at 38 to 84 % of the DOC; the neutral fraction (sum of hydrophobic and hydrophilic neutrals) ranged from IO to 55% and the base fraction (smn of hydrophobic and hydrophilic bases) was generally less than 10%. Three sorption experiments were conducted to investigate selenite sorption on backfill-core samples. Selenite concentrations in these experiments rangoo from O to 11 mg kl in backfill-core samples. The first experiment determined the selenite sorption capacity of the backfill-core samples with selenite in distilloo-deionized water as a background solution. In the second experiment, selenite in 0.1 M CaCl 2 was used as a background solution. In the third experiment, hydrophobic and hydrophilic acid fractions, isolated and concentrated from groundwater samples, were added to the selenite background solutions. Results of the three experiments indicated that the backfill-core samples have a high sorption capacity for Se (generally, greater than 90% of the added Se) and can sorb as much as 99 % of Se added as selenite. Selenium sorption increased in most samples when 0.1 M CaC1 2 was used as a background solution compared to distilled-deionized water background solution. The increase in selenite removal might have been caused by precipitation reactions or by an increase in positively charged surfaces caused by calcium sorption, or both. A substantial decrease in selenite sorption rates occurred in the presence of individual organic-solute fractions. Hydrophilic acids caused the largest decrease in Se sorption by the backfill-core samples.