The evolution of size/molecular weight (MW) of soot from inception to mature soot was studied by means of size exclusion chromatography (SEC) coupled with on-line UV-visible spectroscopy of soot sampled along the flame axis of ethylene, hexane, and benzene premixed flames. Polystyrene calibration and microfiltration were used for the evaluation of the soot MW and size, respectively. Overall, soot exhibited a wide MW/size distribution peaking in two main regions, well detached from each other. The first corresponds with particles/aggregates having MW > 20 000 u, here called the "particle-size region", and the second one corresponds with molecules in the 100-5000 u MW range called the "molecular-size region". The components in the particle-size regions dominated, particularly downstream of the flames and everywhere in the benzene flame. Two classes of components were evident in the particle-size region: one with d > 20 nm (MW > 59 500 u) and the second class well below 20 nm discriminated by SEC, coupled with microfiltration. Meaningful differences in the relative contributions of these two classes were found to occur along the flames, affected by the fuel aromaticity, and they were signatures of the soot growth process and of the different soot inception mechanisms in aliphatic and benzene flames respectively.