This work is a contribution towards the understanding of the properties of hydrogenated silicon nitride (SiN(x):H) that lead to efficient surface and bulk passivation of the silicon substrate. Considering the deposition system used (low-frequency plasma-enhanced chemical vapour deposition (PECVD)), we report very low values of surface recombination velocity S(eff). As-deposited Si-rich SiN(x):H leads to the best results (n-type Si: S(eff) = 4 cm/s - p-type Si: S(eff) = 14 cm/s). After annealing, the surface passivation quality is drastically deteriorated for Si-rich SiN(x):H whereas it is lightly improved for low refractive index SiN(x):H (n similar to 2-2.1). The chemical analysis of the layers highlighted a high hydrogen concentration, regardless the SiN(x):H stoichiometry. However, the involved H-bond types as well as the hydrogen desorption kinetics are strongly dependent on the SiN(x):H composition. Furthermore, "N-rich" SiN(x):H appears to be denser and thermally more stable than Si-rich SiN(x):H. When subjected to a high-temperature treatment, such a layer is believed to induce the release of hydrogen in its atomic form, which consequently leads to an efficient passivation of surface and bulk defects of the Si substrate. The results are discussed and compared with the literature data reported for the different configurations of PECVD reactors. (C) 2009 Elsevier B.V. All rights reserved.