A new capacitance method was developed to investigate properties of the SiNX:H/c‐Si interface. By comparison of the frequency dependence ofC‐Vcharacteristics ofp‐ andn‐typec‐Si substrate sample pairs, flatband voltage for an intrinsic substrate sample can be determined. Analyses of two kinds of frequency dependence, i.e., the variation of voltage at flatband capacitance and the variation of capacitance at flatband voltage, disclose different aspects of the interface state density with respect to the energy level in the band gap, response time, and ionized state. Donor states are predominant in the investigated interface.