Magnesium-aluminum layered double hydroxide containing polysulfide anions was prepared by a hydrothermal reaction from magnesium and aluminum hydroxides. Thioacetamide was used as the sulfide source. The product exhibits light green color corresponding to the S-2(-) and S-3(-) ions and turns to colorless when the sample was stored in air for several days or heat treated in air at 120degreesC. The color change showed that the occluded polysulfide anions were oxidized to sulfate anion by atmospheric oxygen.