A comparative investigation of the nanostructural vs. electronic properties of DECR plasma deposited carbon nitride and carbon oxide alloys provides new insights in the changes of the sp(2)-clusters and sp(3) matrix resulting from the decrease in the average coordination number, as N and O atoms are increasingly incorporated. This review emphasizes the similar incorporation efficiency of N and O atoms in the carbon matrix as a function of the gas phase mixture (C2H2, N-2) or (C2H2, O-2), the decrease in the alloy density deduced from Nuclear Reaction Analysis and optical measurements, and the better ordering of the sp(2)-hybridized carbon phase evidenced by Raman studies. Opposite variations in the ohmic conductivity sigma(T), apparent activation energy E-ACT and hopping transport parameters (related to the localization parameter N(E-F)gamma(-3)) are found as a function of the (N/N+C) and (O/O+C) stoichiometries. This surprising result gives evidence of some competition between the improved ordering of the sp(2) phase shown by Raman spectra in both alloys (stronger in a-C1-xNx:H with a large influence of aromatic clusters) and the increasing polymer-like character of the sp(3) matrix, dominant in a-C1-xOx:H films where the weaker connectivity of the sp(3) matrix is attributed to two-fold bonded oxygen atoms C-O-C and terminating C=O groups. The role of pi and pi* states localization is discussed in the context of bandtail hopping conductivity. (c) 2004 Elsevier B.V. All rights reserved.