The double insulating layer consisting of anodic oxide grown in H2O2 electrolyte and thermally evaporated ZnS fiIm was formed for HgCdTe metal-insulator-semiconductor (MIS) structure in connection with the improvement of HgCdTe surface passivation. It was found for the first time through transmission electron microscopy (TEM) observation that an interface layer of 15 nm in thickness was formed between ZnS and anodic oxide layer. The chemical compositions investigated by energy dispersive : spectroscopy (EDS) using TEM and XPS measurements of depth composition profile and Zn chemical state indicated that the interface layer is composed of ZnO. It can be deduced that ZnO interface layer was caused by the chemical reactions between Zn atoms adsorbed on the surface of anodic oxide and the oxygen atoms diffusing out to the oxide surface in the initial stage of ZnS evaporation. Also TEM high resolution image showed that the structure of anodic oxide layer was changed from the amorphous state to the nanocrystalline structure of 10 nm in diameter after the evaporation of ZnS. The resistivity of double insulating layer was measured to be about 10(10) Omega cm which was proper as an insulating layer for HgCdTe MIS device. The small optical reflectance of about 75 in the region of 5 mu m showed an anti-reflection effect of the double insulating layer. The measured high frequency C-V curve showed a large shift of Rat band voltage due to the high density of fixed oxide charges, about 1.2 x 10(12) cm(-2). The oxygen vacancies and possible cationic state of Zn in the anodic oxide layer were estimated to cause this high density of fixed oxide charges.