Two critical structural characteristics of soot aggregates, i.e., the primary particle size and the spatial arrangement of primary particles in aggregates, were investigated for laminar premixed burner-stabilized stagnation propane flames with and without toluene addition under similar temperature profiles. To link the structural characteristics of soot aggregates with the dynamic processes of soot formation, the particle mass and mobility size were measured by using a centrifugal particle mass analyzer and a scanning mobility particle sizer respectively, which enabled us to obtain the particle size distributions and the spatially fractal feature or mass-mobility exponents. The results show that for pure propane flames, the size growth is relatively rapid as indicated from the particle size distributions, and soot aggregates are highly branched with bigger primary particles. By contrast, for toluene-added propane flames, soot nucleation is faster, while the size growth of primary particles is suppressed, resulting in more compact arrangement of smaller primary particles in the soot aggregates. For both types of flames, with the increase of flame temperature, the process of soot inception is accelerated, but the production of soot is suppressed due to the thermodynamic reversibility of soot precursors at higher temperatures. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.