The morphology of soot aggregates formed in a laminar coflow ethylene diffusion flame was studied by using the techniques of extractive thermophoretic sampling and subsequent image analysis of transmission electron microscopy (TEM) micrographs. A new image-analysis method, the relative optical density (ROD) method, is described in this paper and was used to obtain the morphological properties of soot aggregates. With this new method, the number of primary particles in each individual aggregate is determined by not only the projected area but also the optical density distribution within the projected area of the aggregate. As such, it attempts to address the three-dimensional nature of the aggregates. This method also calculates an overlap coefficient for each object in the TEM image that can be used to identify non-soot-aggregate structures. Unlike the conventional statistical method for fractal-like aggregate structure determination, ROD is independent of empirical constants based on correlations or numerical simulations. Using this new method, the parameters describing the morphology of soot aggregates generated in a laminar coflow ethylene diffusion flame, such as the geometric mean of the number of primary particles per aggregate, the fractal dimension, and the probability distribution function of aggregate size, were determined. ROD was found to be an accurate and reliable TEM image analysis method for studying the morphology of soot aggregates. Crown Copyright (c) 2005 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.