A critical step in currently accepted models for soot formation in combustion is the dimerization of polycyclic aromatic hydrocarbons as small as pyrene, which is necessary within these models to reproduce correctly the soot particle size distribution. We present experimental measurements on the kinetics of pyrene dimerization performed in low-temperature supersonic flows with photoionization mass spectrometric detection, coupled with theoretical results based on careful consideration of the intermolecular interaction energies, binding energy, equilibrium constant, and intermolecular dynamics. These results demonstrate that the equilibrium of the reaction strongly favors the dissociation of the pyrene dimer at high temperature and that physical dimerization (involving van der Waals forces) of pyrene cannot be a key step in carbon particle formation in hot environments such as flames and circumstellar shells.