A reduced layered tungstic acid with a double-octahedral structure was prepared by acid treatment of an Aurivillius-type Bi2W2O9 in the presence of Sn2+ ions. While the color of the product formed by acid treatment with no Sn2+ ions present, H2W2O7, was yellow, a blue powder was obtained after the acid treatment in the presence of Sn2+ ions. No notable change in the morphology was observed after acid treatment. The X-ray diffraction pattern of the product acid-treated in the presence of Sn2+ ions was very similar to that of H2W2O7. Essentially all the Bi3+ ions were lost upon acid treatment, indicating the occurrence of selective leaching of bismuth oxide sheets in Bi2W2O9. A UV-visible absorption spectrum and XPS analysis demonstrated that the W6+ ions were partially reduced to W5+ ions, and the number of protons in the product was correspondingly 2.4 per [W2O7]. These results suggest the successful formation of a reduced layered tungstic acid, H2.4W2O7.