The main dimeric species present in pyrene pitches produced via both catalytic (with AlCl3) and thermal polymerization of pure pyrene have been identified. Characterization of these species was performed by first isolating dimer-rich pitch fractions either by semi-continuous supercritical extraction (SCE) with N-methyl pyrrolidine-toluene mixtures or by solvent extraction with dichloromethane. These fractions were then separated into their individual species via reverse-phase high performance liquid chromatography (RP-HPLC). Molecular weight and structure information of the eluted species was obtained by a combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI) and UV-vis spectroscopy. By following this strategy, two alternant (i.e., six-membered ring) pyrene dimer isomers, dinaphtho[2,1,8,7-defg:2',1',8',7'-opqr]pentacene and dinaphtho[2,1,8,7-defg:2',1',8',7'-ijkl]pentaphene, were found in both the catalytically and thermally produced pitches. Non-alternant (i.e., containing a connecting five-membered ring) pyrene dimer isomers (cyclopenta[1,2,3-cd:4,5-e']dipyrene and cyclopenta[1,2-a:3,4,5-c'd']dipyrene) were also found in both pitches. Interestingly, small amounts of methylated monomeric and oligomeric species (e. g., methyl and dimethyl pyrene; methyl and dimethyl pyrene dimer, etc.) were detected in the catalytically produced pitch, but were completely absent in both the starting monomer and in the thermally produced pitch. These results indicate that the main difference between the two polymerization methods are the AlCl3-induced methylation reactions that occur simultaneously to dehydrogenative condensation reactions. (C) 2014 Elsevier Ltd. All rights reserved.