A new gate-insulating film consisting of phosphorus oxinitride (PON) was formed on an (n)InP surface by vapour transport technique. The substrate temperature was in the range of 280-350 degrees C. The deposited films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). The interfacial properties of phosphorus oxinitride/(n)InP metal-insulator-semiconductor were investigated. The minimum value of the interface states density distribution (D-it), evaluated from high-frequency capacitance-voltage (C-V) measurement was 1.2 X 10(11) eV(-1) cm(-2) at about 0.48 eV below the conduction band edge of InP.