Experimental measurements of flame structure and soot characteristics were performed for ethene inverse diffusion flames (IDFs). IDFs are considered as an ideal flow field for studying incipient soot because the soot particles do not experience the oxidation process. In this study, an laser-induced fluorescence image clarified the reaction zone of IDFs with OH signal and polycyclic aromatic hydrocarbon distribution. A laser scattering technique also identified the soot particles. To address the degree of soot maturing, the carbon to hydrogen ratio and morphology of the soot sample were investigated. From the measurements, the effect of flow residence time and temperature on soot inception could be estimated, and more details on soot characteristics in the IDFs were determined according to the fuel dilution ratio and air/fuel ratio. Fuel dilution results in a decrease of temperature and an enhancement of residence time, but the critical dilution mole fraction is found for which temperature does not effect soot growth. Soot inception is also found to be weakly dependent on temperature as influenced by fuel dilution. (C) 2004 Elsevier Ltd. All rights reserved.