The effect of ultraviolet (UV) irradiation on the characteristics of metal-insulator-semiconductor (MIS) structures with various insulator layer thicknesses has been investigated using the methods of dark and light current-voltage, capacitance-voltage characteristics and IR spectroscopy. The observed alteration of diode parameters due to the UV irradiation is shown to involve interface reconstruction: the insulator layer thickness, its composition and the surface state density. The assessment of their contribution to the exhibited device parameters shows that the initial insulator thickness plays an important role. The most probable origin of interface transformation is discussed.