Experiments on the pyrolysis of C2H2/Ar and C6H6/Ar mixtures with addition of H-2, O-2, and CH4 have been carried out behind reflected shock waves at temperatures ranging from 1400 to 2600 K. Soot formation was measured by laser extinction at 633 nm. Time-resolved temperature measurements were performed via two-color CO absorption on the P(8) and R(21) lines at 2111.54 and 2191.50 cm(-1) using quantum-cascade lasers. For this purpose, 0.5-0.8% CO was added to the gas mixtures. The measured temperature dependence of soot formation in experiments with added O-2, and CH4 was corrected for the temperature effect caused by the thermochemistry of either endothermic pyrolysis or exothermic oxidation or reactions that cause time-dependent deviation from the initial frozen-shock temperatures. In all mixtures, the addition of H-2 resulted in a noticeable decrease of the soot yield. A considerable increase in the soot yield was found with addition of methane to acetylene mixtures. In contrast, in benzene mixtures, the addition of methane caused a decrease of the soot yield. The qualitative analysis of the kinetics of the gas-phase stage of the pyrolysis reactions elucidated the influence of all investigated additives on the change in the key routes of initial stages of PAH and soot formation. We observed that the addition of H-2 to acetylene inhibits the initial stages of the pyrolysis reaction, while the addition of CH4 and O-2 opens up new ways for the formation of benzene and phenyl and following growth of pyrene. In contrast to that, in benzene all the additives studied lead to the suppression of the kinetics pathways for the formation of pyrene and the subsequent growth of soot. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.