The effect of palladium acetylacetonate on the pore structure evolution of isotropic petroleum pitch-based activated carbon fibers (ACFs) is characterized by comparing the pore structure evolution of ACFs that have been prepared from pure pitch and from palladium acetylacetonate-containing pitch. The pore structure was interpreted by applying chi-theory, Brunauer-Emmett-Teller (BET) surface area analysis, Barrett-Joyner-Halenda (BJH) methodology, t-plots, adsorption potential distribution (APD), and nonlocal density functional theory (NL-DFT) to experimental N-2 adsorption isotherms. Pore size and pore volume calculations from chi-theory are in agreement with those from APD and NL-DFT, respectively, whereas, those from the BET, BJH, and t-plot methods are not. However, chi-theory underestimates the total surface area. The validated porosity and surface area results, pore size distribution, and APD were then studied as a function of burnoff value. The pore structure evolution analysis of both types of ACFs showed that the addition of palladium acetylacetonate to the pitch, prior to fiber formation, causes (i) the formation of macropores, (ii) a small increase in microporosity during the early stages of activation, and (iii) increased mesoporosity at burnoff values of > 60%. The presented data and analysis provide a new understanding of the porous structure of novel pitch-based activated carbon adsorbents and potential hydrogen storage materials.