The paper reports the results of a kinetic modeling and thermodynamic analysis of non-catalytic steam reforming (NCSR) of methane and C2–C4 hydrocarbons at 1400–1800 K. The hydrocarbon-to-syngas conversion sequence and the time periods of the major process steps were identified. The initial step consists of hydrocarbon pyrolysis to acetylene and H2 with essentially no involvement of H2O. Noticeable H2O conversion starts at a significantly later point than thermal hydrocarbon conversion, under the effects of radicals formed from the pyrolysis. The H2O conversion results in the generation of OH● radicals, which subsequently react with acetylene to form CO and H2. The key step in the NCSR of C1–C4 hydrocarbons, as well as in their high-temperature interaction with CO2 (carbon-dioxide conversion) is conversion of the acetylene formed from the hydrocarbon pyrolysis. The study findings are important for the optimization of high-temperature syngas production via partial oxidation of hydrocarbons.