The effect of H-2 and C2H2 addition on particle formation in the pyrolysis of C3O2/Ar mixtures was studied behind reflected shock waves. An existing reaction mechanism for the pyrolysis of highly-diluted C3O2 in argon was expanded to conditions with higher C3O2 concentrations (up to 33 volume%) at elevated pressures and high temperatures and was validated against experimental data. The simulations for the gas-phase chemistry were performed with the program CHEMKIN. The heterogeneous particle formation was modeled by post-processing using the program PREDICI relying on the Galerkin method. It was found that in C3O2/H-2/Ar pyrolysis, the induction times and rate constants of particle formation do not differ significantly from those of pure C3O2/Ar pyrolysis. However, the presence of H-2 reduced the particle volume fraction, the mean diameter of particles, the particle number density, and the maximum temperature rise of the mixture. Hydrocarbon-bonded hydrogen in C3O2/C2H2/Ar pyrolysis caused significantly increased induction times for particle formation, decreased particle volume fractions, and decreased temperature rises. The different reaction channels for carbon particle formation were identified in view of the role of hydrogen. An alternating reaction channel including C-2 species played an important role in forming polycyclic aromatic hydrocarbons (PAH) in the mixtures. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.