Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs) are two classes of extremely toxic compounds produced and emitted into the environment as a result of combustion processes. Although no standard method for sampling or analysis of PCDDs and PCDFs currently exists, a method was drafted in 1984 for the purpose at an Environmental Standards Workshop sponsored by the American Society of Mechanical Engineers (ASME), Department of Energy, and U.S. Environmental Protection Agency (EPA). With the method, a Modified Method 5 (MM5) sampling train is used to collect samples for subsequent analysis by gas chromatography/mass spectrometry (GC/MS). The 'dioxin' method has been used during the U.S. EPA National Dioxin Study program for sampling and analysis of the combustion effluents from incineration processes. Validation of the stack sampling technique and the overall analytical method was needed to ensure the quality of the analytical data collected by use of the method. The overall method precision and accuracy also needed to be documented. The authors conducted a program to further develop and validate the method, and the authors evaluated the precision and accuracy in their study. The correct operational conditions and the QA criteria are presented in the article.