Passivation treatment on indium-doped Hg0.8Cd0.2Te epitaxial layers grown on p-Cd0.96Zn0.04Te substrates by molecular beam epitaxy has been performed in order to improve the surface stability of the Hg0.8Cd0.2Te layers. Room-temperature capacitance-volta-e measurements clearly revealed metal-insulator-semiconductor (MIS) behavior for the Al/ZnS/passivated H0.8Cd0.2Te layer/Cd0.96Zn0.04Te diodes. The fast state density and the fixed charge density of the AI/ZnS/passivated Hg0.8Cd0.2Te/Cd0.96Zn0.04Te diode with a sulfur-treated Hg0.8Cd0.2Te layer were smaller than those with a chemically oxidized Hg0.8Cd0.2Te layer. The interface state density at the ZnS/sulfur-treated Hg0.8Cd0.2Te interface were low at at the middle of the Hg0.8Cd0.2Te energy gap. These results indicate that the Hg0.8Cd0.2Te epilayer is significantly passivated by sulfur treatment and that the passivated Hg0.8Cd0.2Te layers can be used for Hg1-xCdxTe-based MIS diodes and MIS field-effect transistors.