How the self-preserving distribution for soot aggregate size evolves as the aerosol passes through the transition regime from free molecule to diffusion limited collision dynamics is experimentally studied over the range of fractal dimension from 1.9 <= D-f <= 2.5. To isolate coagulation from the nucleation and surface growth processes that normally coexist in the flame, soot from a premixed ethylene flame is rapidly sampled and cooled, and introduced into a residence time flow tube. At set distances downstream (corresponding to 0.5-20 s) particles are again sampled, diluted, and their size distribution recorded using a differential mobility analyzer (DMA). These measurements show that a self-preserving distribution is quickly established, but its shape varies as a function of the equivalence ratio and height in the flame from which the soot is sampled. A kinetic model based on the Smoluchowski equation, and employing collision kernels modified to describe the motion of fractal-like particles in the transition regime, reproduces very well both the shapes and temporal evolution of the soot size distributions. The model indicates that the fractal dimension decreases from D-f = similar to 2.5 for soot sampled from near the flame front of relatively lean flames (Phi = 2.0) to a more conventional value of D-f = 1.9 for heavier sooting flames (Phi = 2.4). Thermal desorption experiments and measurements of soot effective density suggest that these trends arise from a decrease in the sentivolatile content of the soot present in the residence tube. (c) 2006 Elsevier Ltd. All rights reserved.