To investigate the characteristics of incipient soot particles with respect to oxygen mole fractions in oxidizer streams, experimental measurements of soot zone structure and degree of carbonization were performed for ethene in inverse diffusion flames (IDFs). IDFs are alternative flames used for research with incipient soot particles because the soot in IDFs does not experience the oxidation process significantly. In the present study, polycyclic aromatic hydrocarbons (PAHs)-planar laser induced fluorescence (PLIF) signals and soot scattering images were used to clarify the soot zone structure of IDFs. The flame height was measured by OH-PLIF with respect to the oxygen mole fraction in the oxidizer stream. The flame height decreased slightly as the oxygen mole fraction in the oxidizer increased. The two-color pyrometry showed that soot temperature along the soot growth path below the flame height was between 2000 K and 2200 K regardless of the oxygen enrichment. Beyond the flame height, soot formation in IDFs remained effective until the temperature maintained a sufficiently high level. The carbon to hydrogen (C/H) ratio, which was obtained through the element analysis of soot sampling, was measured to assess the degree of carbonization. The C/H ratio increased with respect to residence time at which the soot mostly resided in the region below the flame height. In addition, the C/H ratio increased with regard to the oxygen enrichment in the oxidizer. However, regardless of the oxygen enrichment, the C/H ratio increased linearly according to the modified residence time, which considered inception region and high temperature region beyond the flame height. The morphology of the soot sample was investigated by transmission electron microscopy (TEM) micrographs. A micrograph typical of the young soot particles was detected when the soot particles had a C/H ratio of about 2. (C) 2012 Elsevier Ltd. All rights reserved.