Thin films of hydrogenated silicon nitride alloys, a-Si:N:H, have been produced by rf glow-discharge in a SiH 4-N 2 mixture. The samples have been characterized by infrared absorption spectroscopy. The nitrogen concentration in the films has been estimated from the integrated absorption of the Si-N asymmetric stretching band at ∼850 cm −1. The optical band gap was deduced from optical transmission measurements. In this paper, the analysis is focused on the Si-H and N-H stretching bands in the infrared spectra. In a series of samples exhibiting these two features, the Si-H stretching band varies in the range 2105–2230 cm −1, while the N-H stretching band lies in the range 3315–3360 cm −1. The peak frequencies of these bands increase with the nitrogen concentration in the films (0.38 < N/Si < 1.27), as well as with the optical gap (1.85 < E g < 2.42 eV). The frequency shifts of the Si-H and of the N-H stretching modes have been interpreted on the basis of the chemical induction model. Partial charge calculations have been made using an approach which combines the random bonding model for the nearest neighbors with average matrix effects for the more distant neighbors. The local electronegativity for a given network composition was taken as the statistical mean of the local electronegativities associated with the expected local atomic arrangements. The N-H stretching frequency was found to correlate with the partial charge on the N atom in the N-H bond. The partial charge on the Si atom in the Si-H bond and the partial charge on the N atom in the N-H bond have been calculated as a function of the compositional ratio N/Si. The results are consistent with the greater sensitivity of the peak frequency of the Si-H band to variations in the nitrogen content when compared with the peak frequency of the N-H band. The present study shows that the so-called chemical induction model, which has been currently used for the Si-H group, can be also useful for the analysis of changes in the frequency of the N-H stretching mode in amorphous silicon alloys.