The effect of molecular hydrogen on the formation of molecular carbonaceous species important for soot formation is studied through a combination of shock-tube experiments with high-repetition-rate time-of-flight mass spectrometry and detailed chemistry modeling. The experiment allows to simultaneously measure the concentration-time profiles for various species with a time resolution of 10 mu s. Concentration histories of reactants and polyacetylene intermediates (C2xH2, x = 1-4) are measured during the pyrolysis of acetylene with and without H-2 added to the gas mixture for a wide range of conditions. In the 1760-2565 K temperature range, reasonable agreement between the experiment and the model predictions for C2H2, C4H2, C6H2, and C8H2 is achieved. H-2 addition leads to the depletion of important building blocks for particle formation, namely of polyacetylenes due to an enhanced consumption of important radicals by H-2, which are required for the fast build-up of carbonaceous material. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.