For the first time, the photoanodic behavior of undoped n-InP (≈ 1015 atoms.cm−3) is studied in liquid ammonia at low temperature (-55 °C) under atmospheric pressure. Under illumination, the low doping level provides unique conditions to monitor charge transfer at the interface since only photo-generated holes are involved in the passivation mechanism. Specific photoanodic transient phenomena are observed due to the photo-holes availability at the interface which depends on light intensity. Cycle voltammograms differ strongly from the initial ones in the dark. Photo-electrochemical transformations of InP responses are followed by cycle voltammetry for two conditions of illumination; under low and high luminous flux intensity. In spite of a pure photo-holes process for both cases, voltammograms are highly dependent on light intensity while impedance measurements present the same strong modifications. One-volt positive shift of the flat band potential is indeed observed. However, whatever is the luminous flux intensity, a low photo-anodic charge is consumed (≈ 1 mC.cm−2) and the same ultra-thin film of polyphosphazene onto n-InP is shown by XPS analyses. Properties of this new interface is discussed in term of energy diagram and photo-holes transfer ability. Under these experimental conditions used, (J ≤ 100 µA.cm−2 and Q ≈ 50 mC.cm−2), the passivated interface behaves as a photoanode toward ammonia oxidation since the ultra-thin film is maintained onto InP.