The results of experiments on the isothermic pyrolysis of acetylene, benzene, and diacetylene in a flow reactor near a low-temperature threshold of soot formation are presented. Diacetylene showed a much higher ability to form soot, coke, and tar than the other hydrocarbons. The threshold temperature of soot formation from diacetylene (800 K) was found to be lower than the threshold temperatures for benzene (1230 K) and acetylene (1200 K) for the same pyrolysis time (0.17 s) and equal hydrocarbon concentrations (on the basis of C atoms). The induction periods of soot formation for acetylene and benzene at 1100–1200 K, which were estimated from experiments, correlated well with literature data extrapolated from the high-temperature region. Invisible soot particles (0.3-0.5 Μm) and particles at different steps of carbonization were found among the products of low-temperature pyrolysis. Experimental data were analyzed and compared within the framework of two soot formation theories presented in the literature (the “acetylene” and “aromatic” theories). The contribution of the process of polyyne polymerization in a gas phase to the formation of a soot aerosol is discussed.