This work studied the effect of potassium chloride (KCl) on soot formation during the combustion of municipal solid waste (MSW). The nanostructural and oxidative properties of soot particles were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) spectroscopy, Raman spectroscopy (RS), and thermogravimetric analysis (TGA). HRTEM images showed that, under the same combustion environment, the KCl-loaded fuel samples generated soot particles with smaller particle sizes and more compact morphologies. Quantification of the nanostructure further indicated that, with increasing potassium concentrations from 1 to 5%, the fringe separation distance and fringe tortuosity of soot particles slightly increased, yet no clear trend was observed in the fringe length. Raman spectral analysis verified the fringe analysis, suggesting that the addition of KCl made the soot nanostructure more disordered with a larger separation distance and higher fringe tortuosity. In addition, oxidation profiles determined that the reactivity of K-doped soot was enhanced because of a decrease in the apparent activation energy (E a) from 167 to 124 kJ/mol. Moreover, E a showed strong linearity with the fringe tortuosity and fringe separation distance, with R 2 values of 0.91 and 0.97, respectively. On the basis of the experimental data, a schematic of four K–soot interaction routes is proposed to better understand the soot formation during the combustion of solid wastes.