Current-voltage (I-V-g), capacitance-voltage (C-V-g) and photoluminescence intensity variations versus bias voltage (I-p-V-g) are carried out to study Schottky and UV oxidized Schottky diodes realized on n-type indium phosphide. It is shown that the electrical properties of the two types of structures are comparable and are governed by a thin interfacial layer even in the case of the non-oxidized structures. The photoluminescence measurements however led to a net distinction between the two types of structures. The results show that even in the initially oxidized structures, the thickness of the interfacial oxide is greater than expected probably due to further oxidation during the long storage in air. The photoluminescence results show that these samples exhibit a MIS, rather than a Schottky behaviour. Finally, the interface states density has been estimated from the photoluminescence intensity variations versus applied voltage.