Soot formation/growth processes include fuel pyrolysis reactions resulting in PAHs, the coalescence/condensation of PAHs and other fuel pyrolysis intermediates ultimately leading to soot. Here simultaneous LIF-LII images identify the fuel pyrolysis (PAH containing) and soot containing regions along the axial flow streamline within a laminar ethylene/air diffusion flame. Spectrally resolved emission, excitation wavelength dependence and temporal decay characterize and identify the LIF and LII signals within the simultaneous LIF-LII images. The boundary between the PAH and soot containing regions is postulated to mark the onset of carbonization of condensed/coalesced soot precursor material. Support for this postulate is based on the observed photophysical changes resulting in a decline in the LIF intensity and emergence of LII signal along the axial streamline between 33 and 34 mm HAB. Bright and dark field transmission electron microscopy images of material sampled from the axial centerline at this axial height confirm the interpretation of the in-situ optical measurements by revealing the initial transformation of soot precursor material towards solid carbonaceous soot.