The condensed-phase combustion zones of double-base rocket propellants formulated with and without lead salt-burning modifiers have been investigated by using x-ray photoelectron spectroscopy (XPS). Combustion residues from samples extinguished at selected combustion pressures in the range of 4–12 MPa were investigated to a depth where all signs of reaction disappear. The quenched sample of the noncatalyzed propellant was altered to a depth of 10–12 μm only. It consisted of the original propellant and its combustion intermediates like nitrite and oxime. No soot formation was evident. In contrast to that, the samples of the catalyzed propellants quenched at different pressures were altered to the depth of 30–40 μm and a fluffy soot with a large concentration of lead in it was the principal constituent. The lead accumulation in the soot showed a maximum at a depth of about 10 μm below the surface and marked the region of highest catalytic activity. X-ray-stimulated electron spectroscopy revealed that lead was present both as metal and oxide and that more metallic lead was present in the samples from the super burning regime than those of the mesa regime. The Pb C ratio in the soot, produced at different pressures, showed a sudden decrease coinciding with the onset of the mesa region. Thus the onset of the mesa regime can be ascribed to the loss of lead in the soot. The catalyzed propellant did not show the oxime-like reduced intermediate product, signifying that the presence of lead promotes thorough oxidation. The present work shows that modification of the combustion reaction in the catalyzed propellant begins in the condensed phase.