Single-laboratory testing of RCRA Method 8280 for the analysis of chlorinated dibenzop-dioxins and dibenzofurans has been initiated on sample matrices including pottery clay soil, a Missouri soil, a fly ash, a still bottom from a chlorophenol-based herbicide production process, and an industrial process sludge. The analytical method was intended for use in the determination of chlorinated dioxin and dibenzofuran homologs with four, five, or six chlorine atoms per molecule. Revisions to the method that were found necessary for satisfactory analytical performance have been developed and have been incorporated into a revised version of the method. Single-laboratory testing of method 8280 with minor revisions demonstrated satisfactory performance for the target analytes on soil and fly ash samples. Further modification and elaboration of sample cleanup procedures were necessary for analysis of the still bottom and industrial sludge samples.