UV-vis spectra of solid state H3PMo12-xO40 (x=0, 3, 6, 9, 12) catalysts were obtained to elucidate the effect of crystalline water molecules and to explore the reduction potentials of the heteropolyacid (HPA) catalysts. The UV-vis spectra of HPA catalysts in the solid state varied in a systematic way depending on the number of crystalline water molecules (depending on the treatment temperature). Absorption edges of solid HPA catalysts shifted to longer wavelength (to lower edge energy) with decreasing the number of crystalline water molecules (with increasing treatment temperature). Absorption edge energies of H3PMo12-xWxO40 catalysts treated under consistent conditions shifted to higher values with increasing the number of (more electronegative) tungsten atoms in the framework. The absorption edge energies of the H3PMo12-xWxO40 catalysts were directly correlated with their reduction potentials. A lower absorption edge energy corresponded to a higher reduction potential of the HPA. The absorption edge energies determined from UV-vis spectroscopy of HPAs in the solid state could be utilized as a correlating parameter for the reduction potentials of these catalysts. (c) 2005 Elsevier B.V. All rights reserved.