The conversion process of an Aurivillius phase, Bi2W2O9, into a layered tungstic acid by hydrochloric acid treatment has been investigated, and resultant H2W2O7.nH(2)O has been fully characterized. The c parameter of Bi2W2O9 (2.37063(5) nm] decreases to 2.21(1) nm in an acid-treated product dried at ambient temperature. The a and b parameters of Bi2W2O9 [a = 0.54377(1) nm and b = 0.54166(1) nm] also decrease slightly to a = 0.524(1) nm and b = 0.513(1) nm in the acid-treated product dried at ambient temperature, indicating structural changes in the ReO3-like slabs in Bi2W2O9 upon acid treatment. Drying at 120 degreesC leads to a further decrease in the c parameter (1.86(1) nm] with no notable change in the a and b parameters [a = 0.5249(2) nm and b = 0.513(2) nm]. The formation of an expandable layered structure is demonstrated by the successful intercalation of n-octylamine [interlayer distance 2.597(9) nm] and n-dodecylamine (interlayer distance 3.56(2) nm]. The compositions of the acid-treated products are determined to be H2W2O7-nH(2)O typically with n = 0.58 for the air-dried product and n = 0 for the product dried at 120 degreesC. As a consequence, the composition of the layer is H2W2O7, and the decrease in the c parameter upon drying is ascribable to the loss of interlayer water. Scanning electron microscopy reveals no morphological change during acid treatment, which strongly suggests a selective leaching of the bismuth oxide sheets as a reaction mechanism. High-resolution transmission electron microscopy (HREM) observation of the acid-treated product shows consistency with a structural model for H2W2O7, derived from Bi2W2O9 through removal of the bismuth oxide sheets and contraction along the c axis. HREM observation also reveals that the WO6 octahedra arrangement changes slightly with acid treatment. A one-dimensional electron density map projected on the c axis for the product dried at 120 degreesC, H2W2O7, shows good consistency with that calculated for the structural model.