An experimental and numerical study was carried out to analyze the effect of hydrogen addition on the fuel stream at various oxygen mole fraction of the oxidizer (oxygen index, OI), with emphasis on soot volume fraction and maturity. A target laminar coflow non-premixed ethylene flame was used as reference flame. Experimental evaluation of soot optical properties was carried out by measurements of two color absorption/emission signals, whereas detailed numerical simulations accounted for hydrogen tracking on the soot surface, allowing to define a H/C ratio that can be interpreted as representative of soot maturity. Results present an overall reduction on soot content and local flame emission with the H2 addition, while the opposite trend is encountered with the increase of OI. Changes are more pronounced close to the reaction zone near the flame edge, rather than the flame center. Under constant heat release rate conditions, temperature is mainly affected by the change of O2, whereas the addition of H2 results in a slight temperature variation. Both species intensify the combustion reactions, increasing the reaction rates and reducing the flame length. This effect is more noticeable for increasing the OI, than for the H2 addition for the cases studied. Carbon-to-soot conversion efficiency is reduced by H2 addition, as the soot oxidation processes become more predominant over soot formation processes, which are also affected by the change of temperature conditions. At 40% H2 addition, the carbon-to-soot efficiency remains approximately constant with the addition of O2. Overall, these trends are well-captured by the numerical simulations, although they underestimate soot volume fraction, specially for high OIs and near the flame wing. Finally, experimental results indicate that H2 prompts the soot maturation process, being more pronounced above 20% H2 addition, for high O2 contents, specially near the flame centerline.