Structural properties of polymer-like amorphous carbon (a-C:H) films grown in a dual-plasma (microwave (MW) assisted radio frequency (RF)) reactor have been investigated. An interesting transition appears as the film density increases due to increasing ion energy: (1) in situ infrared ellipsometry shows a transition in the C–H bonding modes corresponding to carbon skeleton cross-linking, (2) a quenching of the photoluminescence (PL) efficiency is observed when film density is >1.3 g cm−3. To analyze the origin of such an evolution, X-ray diffraction investigations have been performed on a-C:H powders with densities increasing from 0.9 to 1.7 g cm−3. This paper reports on X-ray measurements performed in transmission geometry using the high photon flux at LURE facilities. The data show differences between the samples in the structure factor, S(K), and in the pair correlation function, G(r). X-ray spectra have pre-peaks in the vicinity of 1 Å−1, which change in position and shape with the density of the powders. The contraction of the first distances, C–C, is observed as the density increases, from for transparent and soft films to for the denser one. The evolution of C-skeleton structure is described in terms of local co-ordination, in relation with the C–H bonding.