Iron coprecipitation was used in bench-scale tests to remove selenium from stripped sour water generated by two petroleum refineries. Chlorine dioxide and hydrogen peroxide were found to convert selenocyanate in the stripped sour water to selenite, which can be removed by iron coprecipitation. An iodometric titration procedure was developed to determine the required oxidant dose. Iron coprecipitation reduced selenium concentrations by 40 to 99 percent in stripped sour water after chlorine dioxide pretreatment. Removal was less effective with hydrogen peroxide as the oxidant: total selenium concentrations were reduced by 28 to 92 percent in stripped sour water after hydrogen peroxide pretreatment. Highest removals were obtained at the highest oxidant and iron doses. Sludges produced in coprecipitation tests were hazardous under California regulations. Ozone oxidized selenocyanate but prevented ferric hydroxide precipitation or coagulation. Air was ineffective at selenocyanate oxidation. Repeatedly contacting iron hydroxide with stripped sour water pretreated with hydrogen peroxide, in a simulation of countercurrent adsorption process, increased the selenium adsorbed on the solids from 32 to 147 ug selenium per mg of iron, but some of the adsorbed selenite was oxidized to selenate and desorbed back into solution.