Direct observation of the chemical evolution of flame-generated soot was demonstrated for the first time using real-time aerosol mass spectrometry. The data presented unambiguously show the transformation of particle phase polycyclic aromatic hydrocarbons (PAHs) into carbonaceous soot containing fullerenes. The mechanism of fullerene formation in the particle phase that is revealed is not consistent with conventional gas-phase formation mechanisms. This mechanism suggests the possibility of a controlled condensed phase synthesis of fullerenes.