Soot volume fraction and dispersion index measurements were performed by multi-color pyrometry in a series of highly controlled ethylene counterflow diffusion flames in which the peak temperature was varied in a 300K range, while maintaining constant stoichiometric mixture fraction and global strain rate. In so doing, the temperature-convective time history remained nearly constant, when the temperature was normalized with respect to its peak value. Therefore, the peak temperature was isolated as the key controlling parameter. We observed two distinct trends in the soot measurements: a high-temperature, high activation energy process near the flame front, for temperatures larger than 1600K, that can be rationalized with conventional nucleation and surface growth models; and a low temperature zero-activation process that is hypothesized as dimerization (homogenous nucleation) of aromatics. The first process is accompanied initially by large values of the dispersion index, denoting small C/H ratios, followed by a rapid decrease towards a plateau, as soot carbonization occurs. The second process is accompanied by a sharp increase in the dispersion index, consistent with a new inception process that is well distinct from that in the high temperature region. In one of the flames studied the soot measurements supplement an extensive set of measurements of gaseous species from a previous study, thereby offering a comprehensive database for modelers. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.