Abstract <p>This paper presents a numerical study of the effect of hydrogen additives on soot formation in the pyrolysis and oxidative pyrolysis of methane and ethylene in reflected shock waves. In the pyrolysis of methane, hydrogen additives significantly decreased the yield of soot in the entire test temperature range of 1700–2800 K. In the oxypyrolysis of methane, a weak promoting effect of hydrogen additives on soot formation was found at low temperatures (1600–1800 K), whereas the additives of H₂ noticeably inhibited soot formation at higher temperatures. In the pyrolysis and oxypyrolysis of ethylene, hydrogen additives strongly inhibited soot formation over the entire test temperature range of 1500–2300 K.</p>