Acetylene chemistry is studied by means of threshold ionization mass spectrometry (TIMS) in remote Ar/C2H2 expanding thermal plasma to identify the growth precursors of hydrogenated amorphous carbon (a-C:H) films. More than 20 hydrocarbon species are measured, enabling a comprehensive study of acetylene chemistry in the plasma environment. It is shown that the plasma composition is controlled by the initial ratio between the acetylene flow into the reactor and argon ion and electron fluence emanating from the remote plasma source. Complete decomposition of acetylene to C, CH, CH2, C2, and C2H radicals is achieved in subsequent charge transfer and dissociative recombination reactions under low acetylene flow conditions. The formation of soft polymer-like a-C:H films can be attributed to C, C2, and also partially to CH and C2H deposition. At acetylene flows higher than argon ion and electron fluence, reactions of C, CH, C2, and C2H radicals with acetylene lead to the formation of various hydrocarbon species, whose behavior is dependent on whether the number of carbon atoms is even or odd. The detected resonantly stabilized C3, C3H, and probably also C5 and C5H radicals are unreactive with acetylene in the gas phase and are, therefore, abundantly present close to the substrate. The C3 radical has among them the highest density, and it is identified as the significant growth precursor of Ar/C2H2 expanding thermal plasma deposited hard a-C:H films.