Real-time aerosol mass spectrometry was used to measure the size and composition of individual soot particles in an acetylene diffusion flame. This on-line data analysis technique permits for the first time direct observation of the evolution of the carbonization process, determination of the degree of carbonization of each measured particle, and measurement of the size distribution of both polycyclic aromatic hydrocarbon (PAH)-containing and mature soot particles. The carbonization process is characterized by rapid exchange of hydrogen between the PAH and pyrolytic addition of small hydrocarbons to form larger PAH molecules. The hydrogen exchange rare builds until carbon-carbon bond rearrangement becomes facile. The structural rearrangement/dehydrogenation process is very rapid once started. Rapid carbonization permits unambiguous size measurement of both PAH-containing and mature soot particles. Rapid hydrogen exchange yields a low activation energy path for making radicals in the particle phase and permits the PAH-containing media to act as a hydrogen sink. The presence of substantial amounts of labile hydrogen in the PAH-containing particle is demonstrated by the presence of preferentially hydrogenated PAHs. The absence of ethynylated PAHs and the presence of the hydrogenated PAI-Is are the result of flame pyrolysis of the PAH-containing particles. Optical images of flame particles collected by an independent sampling method conclusively confirm the presence of the micron sized PAH-containing particles in the flame and strongly suggest that mature soot aggregates are formed directly from the micron-sized PAH-containing particles. The optical images and the size distribution data cast doubt on the currently accepted mechanisms for mature soot aggregate formation. A new mature soot aggregate formation mechanism found in the aerosol literature is presented. This mechanism readily explains our results and many of the universal characteristics of soot. The implications of these measurements are discussed. Published by Elsevier Science Inc.