A simple method for the determination of the mean interface density D itm of MIS structures is proposed. The method is based on modified bias–thermal stress measurements and combines capacitance–voltage measurements with doping profile determination. The method is illustrated on InP MIS structures with boron nitride (BN) as an insulator. The deposition of the BN films was performed at low temperature using a microwave plasma-enhanced CVD system. The method is validated by comparison to deep level transient spectroscopy results obtained on the same test samples.