Detailed numerical simulations are presented of laminar microgravity spherical diffusion flames at their experimentally observed sooting limits. Ten normal and inverse flames fueled by ethylene are considered. Observed in a drop tower, these flames were initially sooty but reached their sooting limits 2 s after ignition (or slightly before). The flames span broad ranges of stoichiometric mixture fraction (0.065-0.692), adiabatic flame temperature (2226-2670 K), and stoichiometric scalar dissipation rate (0.013-0.384 s(-1)). They were modeled using a one-dimensional, transient diffusion flame code with detailed chemistry (up to toluene) and transport. Radiative losses from products were modeled using a detailed absorption/emission statistical narrow-band model coupled with a discrete-ordinates method. Flame structure at the sooting limits was examined, emphasizing the behavior of carbon to oxygen atom ratio, temperature, and scalar dissipation rate. For ethylene flames with sufficiently long flow times it was found that soot formation coincides with regions where the C/O atom ratio and temperature exceed critical values, specifically 0.53 and 1305 K. respectively. The scatter about these critical values is small, which is noteworthy considering the wide range of flame conditions. These observations are consistent with the expected effects of H radicals on the propargyl soot pathway. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.