The evolution of the nascent soot particle size distribution function (PSDF) was determined by mobility sizing for two series of atmospheric pressure premixed ethylene flames in the burner stabilized stagnation flame configuration. The first series of flames has an equivalence ratio of 1.8, corresponding to conditions just above the sooting limit. The second series has an equivalence ratio of 2.5 and is quite heavy in soot production. In each series, six flames were tested in which the cold gas velocity is varied to obtain flame temperatures ranging from 1559 to 1941 K. The temperature profiles were carefully determined and the comparison to pseudo-one dimensional simulations was satisfactory. It was found that the evolution of the PSDFs with respect to flame stoichiometry, temperature and growth time is consistent with the understanding of Idnetic competition during soot formation. Finite rate kinetic limitations are observed at lower temperatures and thermodynamic reversibility occurs at higher temperatures. The observed PSDF features are highly sensitive to competition among the various processes of soot formation, from nucleation to coagulation and gas-surface reactions. The PSDFs are mostly bimodal with both nucleation and coagulation mode particles present. The evolution of the PSDF indicates a strong contribution to the mass of coagulation-mode by the nucleation-mode particles. The measured PSDFs offer comprehensive, canonical data sets useful for testing models of soot formation. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.