The interactions of H2O with solid H3PW12O40 have been studied by TR spectroscopy, and the results are compared with those obtained for H2O interacting with other acidic or superacidic solid systems (acidic zeolites and the perfluorosulfonic membrane H-Nafion). In fully dehydrated HPW a heterogeneous family of hydroxyls is evidenced which, by interaction with water, leads to the formation of neutral 1:1 H-bond adducts. At higher water pressures, the formation of symmetric diaquahydrogen ions, and of clusters of protonated water is also observed. The H2O...H+...OH2 symmetrical adducts are characterized by a very broad absorption in the 1400-700 cm(-1) range, which, by comparison with theoretical studies on gas-phase dusters and with experimental results obtained on H5O2+ salts, is assigned to the vO...H+...O mode. On the contrary, the IR spectra of fully dehydrated HPW do not show any absorption in this range.