The IR and UV-vis diffuse reflectance spectra of mesoporous WO3/ZrO2 are discussed and a scheme proposed for the acid site structure on the WO3/ZrO2 surface. The high Brbnsted acidity of WO3/ZrO2 (H-0 <= -11.38) can be explained by the formation of large zirconium polytungstate species such as [ Zr61/2+W66+O202-](-) H+ on the WO3/ZrO2 surface.