Soot is one of the degradation products of material burning, having the fingerprints of the conditions in which it is formed. In this work, in-flame soot was probed from flaming combustion of balsa core and its sandwich composite at different heat flux scenarios during mass loss cone calorimeter tests. Soot probing was performed by thermophoresis. Electron microscopy was performed to analyze the size of the particulate media at multiscale. The size of the aggregates and the primary particles were found to be inherent to scenarios, i.e. materials specifications and heat flux rates. Nanoscale structure of in-flame soot was consistent with the results of thermogravimetric analysis of emitted-deposited soot. This semi-quantitative study contributes to soot observations in fire scenarios and constitutes the first application of soot probing by thermophoresis in a bench-scale fire scenario simulated by cone calorimetry. Technique shall be used in future to support emitted soot and smoke data.