Ring-opening conversion of multinuclear aromatics can be used to upgrade heavy aromatic oils to lighter products, and it is usually performed reductively with H-2. Oxidative ring-opening is an alternative strategy that involves three steps: (i) oxidation of multinuclear aromatics to quinonoids, (ii) further oxidation and ring-opening, to produce aromatic carboxylic acids, and (iii) decarboxylation of aromatic carboxylic acids. In the last step, decomposition by ketonization is an undesirable side reaction that leads to a ring-closed product. Selectivity control during aromatic carboxylic acid decomposition was investigated using biphenyl-2-carboxylic acid, biphenyl-2,2'-dicarboxylic acid, zinc(II) biphenyl-2-carboxylate, and zinc(II) biphenyl-2,2'-dicarboxylate. The reaction networks of thermal decomposition of the aromatic carboxylic acids were determined. Decomposition of biphenyl-2-carboxylic acid took place mainly by decarboxylation to produce biphenyl; dehydration and ring-closure to produce fluorenone, and the formation of diphenic anhydride as intermediate product leading to fluorenone. Decomposition of biphenyl-2,2'-dicarboxylic add proceeded through decarboxylation to biphenyl-2-tarboxylic acid as intermediate, as well as two seemingly related pathways, leading to the formation of a hydroxy-fluorenone and a cyclic trione. Over the temperature range from 340 degrees C to 400 degrees C, thermal decomposition invariably resulted in a higher ketonization than decarboxylation selectivity. Decomposition of the analogous zinc carboxylates demonstrated that ketonization could be suppressed and the most abundant products were biphenyl > fluorenone > fluorene. It was possible to achieve a biphenyl (decarboxylation) to fluorenone (ketonization) selectivity ratio of 17:1 during batch reactor decomposition of zinc(II) biphenyl, 2,2Cdicarboxylate at 380 degrees C. Reaction. stoichiometry indicated that water should affect selectivity, which is consistent With observations in the literature, but this aspect was not investigated further.