This action finalizes revisions to 40 CFR part 60, appendix A, Method 23, 'Determination of Polychlorinated Dibenzo-P-Dioxins and Polychlorinated Dibenzofurans from Stationary Sources,' which was last revised on March 31, 1995 (60 FR 28378). This update to Method 23 is a complete republication of the method to determine polychlorinated dibenzo-p-dioxins (PCDD's) and dibenzofurans (PCDF's) which will now include an option to determine polycyclic aromatic hydrocarbons (PAH's), and/or polychlorinated biphenyls (PCB's). This update revises the analytical procedure to include isotope dilution mass spectrometry combined with high resolution gas chromatography, which is consistent with industry practice. The update moves the method from a prescriptive to a performance-based methodology and removes requirements in the method to use outdated standards or materials. This provides industry with an appropriate method in the execution of Method 23, which has shown wide variation in its implementation, and will allow many performance-based options and procedures.