The hydration state of phosphotungstic acid (H3PW12O40) was explored through the use of in situ Fourier transform infrared (FT-IR) spectroscopy, water sorption microcalorimetry, and density functional theory (DFT) quantum chemical calculations. In addition. the proton affinities of the various sites on the Keggin unit were evaluated. The small differences (< 10 U mol(-1)) between the proton affinities of bridge and terminal oxygen atoms indicate that protons likely reside on both types of sites on the Keggin unit. Vibrational spectra calculated by DFT and measured experimentally by in situ FT-IR spectroscopy compare very,cry well. The adsorption energy of a water molecule on an anhydrous Keggin unit was calculated to be approximately -55 to -70 U mol(-1) for the formation of H3O+, independent of protonic site. The range compares well to the heat of water sorption (-65 U mol(-1)) determined from microcalorimetry on a sample dehydrated at 573 K. The uptake of gaseous water molecules was found to be five times higher on a sample pretreated at 473 K than on one pretreated at 573 K, indicating that the hydrated secondary structure does not reform after the high temperature pretreatment. In situ FT-IR spectroscopy confirms the lack of reversibility during dehydration/rehydration treatment for samples heated to 573 K. (C) 2003 Elsevier B.V. All rights reserved.