Use of hydrocarbon fuels as coolants for future high-Mach aircraft is challenged by the formation of carbonaceous deposits during thermal stressing at high temperatures (>500 degrees C). Three hydrogen donors, tetralin (THN), alpha-tetralone (THNone), and benzyl alcohol (BzOH), and two organic selenides, diphenyl selenide (Ph2Se) and diphenyl diselenide (Ph2Se2), as well as their mixtures, are selected as thermally stable additives to inhibit the deposition from the thermal stressing of n-dodecane and Chinese RP-3 (No. 3 jet fuel). It is found that the amount of solid deposits from thermal stressing of RP-3 is reduced by 77.0% with the additive of Ph2Se2/THN/THNone. The carbonaceous solid is further characterized using temperature-programmed oxidation (TPO), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). It is revealed that hydrogen donor THN/THNone and organic selenides possibly reduce the carbon deposits through retarding the thermal cracking rate, blocking surface catalysis, and depressing reactivity of sulfur with the surface metals, as well as their synergistic effect. The morphologies of deposits also dramatically change after adding organic selenides or hydrogen donors.