High-resolution transmission electron microscopy (HRTEM) has been performed on soot samples collected from two smoking laminar ethylene diffusion flames (one steady and one unsteady) and from the active-flaming region of a 5-m diameter JP-8 pool fire. The motivation for this study is to improve the understanding of the influence of soot microstructure on its optical properties. The soot sampling positions in the steady ethylene flame correspond to locations of maximum soot mass growth, partial soot oxidation, and quenched oxidation along a common streamline. Visual examination of the HRTEM images suggests that the graphitic crystalline layers of soot undergo increased densification along the sampled streamline in the steady laminar flame. Quantitative image analysis reveals a small decrease in the mean graphitic interlayer spacing (d(002)) with increasing residence time in the high-temperature region. However, the differences in the mean interlayer spacing are far smaller than the spread of interlayer spacings measured for any given soot sample. Post-flame samples from the unsteady ethylene flame show interlayer spacing distributions similar to the lower region of the steady flame. The soot samples from the pool fire show little evidence of oxidized soot and have interlayer spacings similar to the unsteady ethylene flame. Previous research in the carbon black field has demonstrated a direct relation between the graphitic interlayer spacing and the optical absorptivity of the carbon. Consequently, the current HRTEM results offer support to recent measurements of the dimensionless extinction coefficient of soot that suggest that the optical absorptivity of agglomerating soot shows only minor variations for different fuels and flame types. (c) 2005 Elsevier Ltd. All rights reserved.