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Pyrolysis of acetylene was investigated in a tubular reactor of graphite with an internal lining of alumina. The temperature range was 850–1650 °C, and the pressure was about 0.133 bar (100 Torr). Pure acetylene and acetylene diluted with argon or hydrogen were used as feed. Carbon and hydrogen are the main products from acetylene pyrolysis particularly at higher conversion. At lower conversion of acetylene, other gas products were formed; the amount of these depended on temperature, dilution, and conversion. Benzene and vinyl acetylene are the main gas products from pyrolysis of pure acetylene below 1000 °C and at low conversion. Diacetylene increases with increasing temperature. Dilution with hydrogen changes the composition of the gas product, decreases the selectivity of vinyl acetylene and benzene, and increases the formation of methane and ethylene. Gas-phase equilibrium may be approached between some components. The conversion of acetylene with argon dilution and low conversion was found to be of second order. Pyrolysis of pure acetylene at lower temperature and low conversion gave the rate constant k = 3.1 × 109 · exp(−34.8/RT) L mol−1 s−1 with an activation energy of 34.8 kcal mol−1. The initial reaction at 864 °C is a molecular formation of vinyl acetylene. The initial activation of acetylene in gas phase seems to be rate determining and of second order in acetylene. Decomposition of acetylene can take place both homogeneously and heterogeneously. Above a critical partial pressure of acetylene, the decomposition is apparently explosive with instant plugging of the reactor with carbon.