Abstract Samples of soot precursor particles and carbonaceous soot from a nonsmoking ethene diffusion flame have been analyzed by means of laser microprobe mass spectrometry (LMMS). The mass spectra of soot precursor particles from the lower flame display many peaks in the 200–300-amu range that are characteristic of polycyclic aromatic hydrocarbons (PAHs). The most prominent of these are the masses 252, 276, and 300, which correspond to the isomers of C 20H 12, C 22H 12, and C 24H 12, respectively. These masses are among those predicted by Stein and Fahr to be the most thermodynamically stable (“stabilomers”) under typical hydrocarbon flame conditions, and they have been previously reported as components of soot collected from a variety of fuels and combustion configurations. Carbonaceous soot from the upper region of the flame yields mass spectra composed of carbon-hydrogen clusters, C x H y with x = 3 to 24 and y usually 0, 1 or 2. Smaller amounts of PAH-like species with masses in the 418 to 444 amu range are also found. These results suggest that the carbonization process observed in this flame is the dehydrogenation of the PAH species formed in the lower flame unaccompanied by polymeric growth. The LMMS technique provides a lower bound value of the hydrogen mole fraction X H of 0.36 ( C H = 1.8 ) for precursor particles from the lower diffusion flame and 0.15 ( C H = 5.6 ) for carbonaceous soot aggregates.