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Acetylene has been pyrolysed in a flow reactor system at 914–1039 K and 6–47 kPa, with residence times between 3 and 50 ms. Pressures at the reactor inlet were greater than those at the outlet because of viscous drag. This permitted calculation of the viscosity of acetylene, which was found to have a dependence on the absolute temperature proportional to T 0.71±0.08 . Expressions were developed to describe the yields of products in a compressible gas undergoing simultaneous viscous expansion and slow reaction. Yields of products were measured by gas chromatography. Vinylacetylene and benzene were found to be primary products, with orders of two and three, and activation energies of 208±13 and 164±17 kJ mol −1 , respectively. Acetone, which occurs as an impurity in commercial acetylene, was found to accelerate the formation of both products by a factor between two and four. This provided evidence to support a free-radical mechanism with a second-order initiation step, whose activation energy is approximately in the range from 220 to 250 kJ mol −1 . The chain length was estimated to be between 10 and 150, depending on the conditions and the identity of the termination step.