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Hydrogenated amorphous carbon and carbon nitride films (a-C1−xNx:H) have been synthesized from methane, acetylene, or acetylene–nitrogen precursor gases using a high density electron cyclotron resonance plasma. The deposition and etching rates, along with the film stoichiometry, density, Raman signature of the sp2 phase, and optical and transport properties, have been studied as a function of plasma parameters (microwave power and negative bias of the substrate). While low-density H-rich carbon films have been grown from methane for ion energies up to 200 eV, films grown using acetylene have been obtained at high deposition rate (1.1 nm s−1) with H content below 25 H at. % and density of 2.0 g cm−3, which makes them interesting as electronic materials. For dense carbon nitride alloys, the maximum (N/N+C)=0.35 is limited by the vanishing growth rate, which results from ion-assisted chemical etching mechanisms. A larger N2 plasma etching rate related with lower film density is observed for (N/N+C) values above 0.20. As a function of the N content, Raman spectra give evidence of a continuous structural ordering of the sp2 phase, which is confirmed by a modeling of the ohmic conductivity σ(T) data based on hopping transport within a bandtail distribution of localized π states. With increasing N content, a better overlap of p orbitals along with an increase in the localization length are expected as a consequence of a less constrained environment. A maximum of the room-temperature conductivity at (N/N+C)=0.28 reveals the onset of a transition towards polymeric alloys with lower mean coordination number.