This paper has investigated the reaction process concerning pyrene formation from fluoranthene in their electronic ground states. Both aromatic compounds are considered as direct soot precursors. The geometrical parameters, the vibrational frequencies, and the zero-point energies have been calculated using the BMK (Boese-Martin for kinetics) method and the 6-311++G(d,p) basis set. More accurate single-point energies have been obtained using BMK/6-311++G(3df,2p) to retrieve thermodynamic properties (Delta H-r degrees(T) and Delta(r)G degrees(T)) over a wide temperature range (298-2500 K). The isomerization reaction of fluoranthene to pyrene is exothermic and spontaneous in standard conditions. The transition states and the possible intermediate species have been located on the singlet potential energy surface in order to determine the reaction mechanism. Two different reaction channels have been investigated and characterized by entrance reaction barriers of about 419 and 771 kJ mol(-1) for the first and the second reaction pathways, respectively. The present work demonstrates that the first reaction channel is the most energetically favored pathway at high temperatures. Therefore, the kinetic parameters of the forward and reverse first step reactions have been determined in sooting flame conditions.