A method of UV-visible spectra analysis was developed for investigating the relationship between the optical properties and the complex composition/structure of carbonaceous species formed in a sooting premixed ethylene flame. The UV-visible spectra were measured on carbonaceous species, caught on quartz plates inserted along the flame and recovered by solvent treatment. The contribution and the structure of components in a very wide molecular weight range, from 200 to 1E11 u, were evaluated by size exclusion chromatography coupled with on-line UV-visible spectroscopy of the dichloromethane-soluble and dry soot fractions. The optical band gap and the UV peak position of each MW-segregated fraction were evaluated by means of a spectral deconvolution procedure, to get details on the carbon network structure in terms of sp(2) and sp(3) sites and size and stacking of the aromatic units. The species included in the 1E8-1E11 u range underwent graphitization/growth reactions during the formation process of carbonaceous species. Large polycyclic aroamtic hydrocarbon (PAH) molecules and clusters of PAH, included in the 200-1E8 u range, did not exhibit molecular growth and internal structure transformation, suggesting that they merely aggregate/coagulate participating to the formation of carbon particles.