The microstructure of distributed electron cyclotron resonance plasma-deposited hydrogenated amorphous carbon films (a-C:H) was investigated using electron diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. Experimental evidence of the existence of transpolyacetylene (TPA) chains in a-C:H films free of nanocrystalline diamond is presented. The values of the mean bond angles and lengths and first neighbor numbers are consistent with the TPA data. The Raman spectra were fitted using the G and D bands and the bands centered at 1140, 1233, and 1475 cm−1 assigned to TPA chains modes. The relative intensity of the latter decreases while hydrogen content decreases. A significant sp2-CH olefinic mode contribution to the infrared stretching band is observed for the low-density films (∼1.2 g/cm3). TPA chains growth is enhanced when ion current density and energy decrease.