A basic discussion drawn from carbon black studies is presented as applicable to understanding soot primary particle structure. In this context, the relation between bright and dark field transmission electron microscopy (TEM) and selected area electron diffraction (SAED) is discussed. The potential for these techniques to characterize the degree of carbonization or crystallite growth of soot is discussed and then applied to soot thermophoretically sampled at different positions along the axial streamline of an ethylene gas-jet diffusion flame. Bright and dark field TEM image pairs are presented for soot thermophoretically sampled at a series of axial heights above the burner and related to the transformation process of coalesced soot precursor material into solid carbonaceous soot. The potential to quantify the degree of carbonization of the soot is examined via SAED patterns. It is found that higher resolution (via HRTEM or in the SAED patterns) is needed to extend these techniques into a quantifiable diagnostic.