The a-SiN1.6:H / a-Si:H interface produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) has been characterized by using metal / a-SiN1.6:H / a-Si:H / c-Si (MNSS) devices with different a-Si:H film thickness dα. The increase of the voltage pulse transient photocapacitance signal as a function of dα allows the separation of interface and bulk traps absorption. Using the previous EPR measurement of bulk states (6×1015cm-3) as a calibration value, a simple model gives the interface (D-) trap density located at 0.8 eV below EC, NINT = 1.5×1011cm-2, and the optical cross section σOPT=6×10-17cm2. The analysis of the MNSS complex admittance as a function of temperature and frequency gives a similar trap density 1×1011cm-2eV-1, with a capture cross section for electrons σC=4×10-15cm2. At the a-SiNx:H / c-Si interface the MNS photocapacitance cross section for these interface defects is apparently lower by one order of magnitude compared to that obtained for MNSS. © 1991 Elsevier Science Publishers B.V. All rights reserved.