The influence of interdiffusion at the interfaces and oxidation of the topmost layer on the polarizing efficiency of the FeCoV/TiZr supermirror has been studied. It was found out that the thickness of non-magnetic interdiffusion regions is 10±2 Å and the thickness of non-magnetic oxide layer on top of the supermirror is 40±5 Å. In spite of these structural imperfections, the polarizing efficiency of supermirrors is still quite high. The mean polarization of the neutron beam after two reflections from magnetized FeCoV/TiZr supermirrors was found to be no less than 0.9983 within 1.3-2 Å spectral range.