Nonlocal gradient corrected density functional theory was used to determine the optimal positions of the protons and their relative affinity in the secondary structure of phosphotungstic acid. First, a body centered cubic structure associated with the hexahydrate was incorporated into the model. The desorption energies of the first and second water molecules from a bridging H 5O 2 + species were calculated to be 109 and 55 kJ mol -1, respectively. The H 3O + or H + species remaining after dehydration are preferentially located between two oxygen atoms of adjacent Keggin units. The preferred position is between two terminal O d atoms or between a terminal O d atom and a bridging O c atom, depending on the alignment of the Keggin units. The energy advantage of sharing a proton between two Keggin units is in the range of 43 to 71 kJ mol -1. Three-dimensional periodic calculations indicate that anhydrous phosphotungstic acid can form a regular lattice in which all protons are shared between the heteropolyanions.