Optical parameters such as as the energy band gap (Eg), and the position of maximum absorption were used to characterize the carbon particulate matter and its evolution along the carbon formation region in methane-oxygen premixed flames. The energy band gap using the Tauc optical band gap model, allowed for the identification of classes of chromophores having different aromatic sizes. The Eg was shown to be correlated with the molecular weight-size distribution and optical properties of batch-sampled particulate separated by size exclusion chromatography and the measured values of the H/C ratio of the particulate matter sampled along the flame. Some changes of the maximum position at 210-240nm were detected along the flame indicating changes in the carbon internal structure in terms of sp3/sp2 carbon hybridization. The particulate matter structure in terms of both Eg and the maximum position showed to be affected by the flame temperature and related to the hydrogen content of the particulate.