Chemical composition of nascent soot in burner-stabilized, premixed ethylene–oxygen–argon flames was studied using micro-FT-IR spectroscopy. The flames have an identical unburned gas composition with equivalence ratio equal to 2.07, but they differ in cold gas velocity. Nascent soot was sampled using a dilution probe over a range of burner surface-to-probe separation. The particles were deposited on thin-film substrates in a cascade impactor at two cut sizes of D50 = 10 and 56 nm. The micro-FT-IR spectra revealed the presence of aliphatic C–H, aromatic C–H and a number of oxygenated functionalities. Spectral analyses were made to quantify variations of the aliphatic and aromatic C–H groups with flame temperature and sampling position. Results show that the aliphatic-to-aromatic C–H ratio is greater than unity in all cases, indicating the presence of an appreciable amount of aliphatic C–H bonds in nascent soot. The relative content of aliphatic components was found to increase with an increase in the maximum flame temperature. To examine the nature of these aliphatic constituents, a thermal desorption–chemical ionization time-of-flight mass spectrometry was used to softly fragment a soot sample and ionize the fragments. Results suggest that the aliphatic constituents are alkylated aromatics with molecular weights spanning from 200 to 900 amu. This work provides definitive and quantitative observations that aliphatics are abundant at early stages of soot mass and size growth.