The presence of soot precursor particles in hydrocarbon diffusion flames is made clearly evident by means of contemporary sampling techniques in combination with transmission electron microscopy (TEM). These particles are the immediate predecessors of mature, partially dehydrogenated carbonaceous soot aggregates, and recent analytical tests indicate they contain a mixture of polycyclic aromatic hydrocarbons. In this work, the conversion rate of precursor particles to carbonaceous soot aggregates is investigated. In one method the time interval for the morphological conversion of precursor particles to aggregates is observed in diluted diffusion flames. Temperature profiles are measured by rapid insertion thermocouple thermometry. This information is supplemented by the observation of the sudden decrease of the hydrogen mole fraction of soot precursor particles as measured by laser microprobe mass spectrometry. The Arrhenius rate constants for the carbonization of precursor particles are derived from these measurements. Illustrations of the use of this data in achieving high combustion efficiencies and reduced soot formation in combustion devices are discussed.