This study reports experimental data regarding the effects of the addition of propene and propane in the fuel stream on soot formation in acetylene counterflow diffusion flames. Compared to the baseline acetylene flame, a continuous reduction of soot volume fraction was observed as more propene was mixed in the fuel. However, the addition of 1% propane resulted in an enhancement of soot formation, exhibiting an interesting fuel synergistic effect. The propane-doped flames were seen to have a higher sooting tendency compared to the propene-doped one when the molar doping ratio was less than 5%. These experimental results were rather unexpected considering that propene has a much higher sooting tendency than propane. Through a kinetic simulation accounting for the formation of molecular soot precursors, it is shown that the previous CH3-dominated reaction routes in forming propargyl radicals, which were typically used to rationalize the synergistic effect of ethylene-based binary fuel mixtures, were insufficient to explain the present observation of propane-doped acetylene flame having high sooting tendency than the propene-doped one at low doping ratios. The role of H radicals in the formation of C-4 species (via the paths C2H2 + H -> C2H3 and C2H3 + C2H2 -> n-C4H5) was found to be critical in understanding the sooting behavior of C2H2 flames. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.