Soot concentrations at different positions through the carbon formation zone of a premixed acetylene/ oxygen flat flame at 20-mm Hg were evaluated by measuring the attenuation of a laser beam passed through the flame. The required soot absorption coefficients were measured in the same optical system by replacing the flame with standard soot samples deposited on glass slides in a molecular beam sampling instrument. The average spectral absorption coefficient of the young soot studied here (5160 cm "1 ) is substantially smaller than the value (76.000 cm "1 ) often used for full-grown soot clusters, which agrees qualitatively with the expected effect of graphitization and decreasing H/C ratio in the aging of soot. In comparison with soot-particle concentrations measured previously under the same flame conditions using molecular beam sampling and electron microscope analysis of beam deposits, the concentrations observed here are substantially larger in the regions of particle nucleation and rapid surface growth, but in good agreement farther downstream in the region of predominant particle coagulation. The early difference is attributed to absorption by large hydrocarbon species whose sizes range up to that of the smallest soot particles observed under the electron microscope (about 15-A diam or 2130 atomic mass units). The lumped concentration prof'fle of the large hydrocarbons, estimated by assuming the absorption coefficient of these species is the same as that of the young soot, exhibits a strong peak immediately after the onset of soot formation in the region of rapidly increasing surface growth rate. Decomposition of the large hydrocarbons is probably catalyzed by soot-particle surfaces, and some of these species may serve as soot nuclei or intermediates.