The 2,3,7,8-substituted polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are among the most toxic compounds known, and several sources of exposure to these chemicals should be monitored to protect human and environmental health. The current predominant method of analysis is too expensive and cumbersome, and comprehensive two-dimensional GC coupled to time-of-flight mass spectrometry (GC x GC--TOF-MS) has the potential to lower the costs and speed analysis of PCDD/Fs. In this study, GC x GC--TOF parameters were evaluated and optimized to yield complete separation of the 17 most important PCDD/F congeners from polychlorinated biphenyls (PCBs) interferences, and to attain the lowest detection limits. The optimization study entailed evaluation of oven temperature programs, column flow rates, ion source temperatures, electron ionization energy, data acquisition rate, and various GC x GC parameters, including modulation period, modulator temperature offset and hot pulse duration. After optimization, all 17 PCDD/Fs were separated in <60 min, and in particular, the critical pair of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and pentachlorobiphenyl congener CB126 did not co-elute chromatographically. Accurate identification and determination of all analytes could be made using their deconvoluted full mass spectra. In GC x GC, the modulation period and start time were the most important factors that affected sensitivity and selectivity for the analysis of the PCDD/Fs. The modulation period should be < or = 4s to preserve separations achieved in one-dimensional GC, and the modulation start time was important to achieve one large slice and two smaller symmetrical slices of TCDD to maximize its detection sensitivity. After optimization, the method could identify 0.25 pg of TCDD with standard injection from its full mass spectrum.