The in situ IR spectra of H3PW12O40.nH(2)O (n = 0-6.2) in the range 4000-1100 cm(-1) were measured, and that of H3PW12O40.6H(2)O (i.e., [H5O2+](3)[PW12O403-]) with planar H5O2+ was investigated. The spectrum showed a broad continuum below 3700 cm(-1), with peaks at 3420, 2720, 2030, and 1640 cm(-1), and was deconvoluted into five bands at 3420, 2720, 2030, 1780, and 1640 cm(-1). These bands, except for 2030 cm(-1), were shifted by the deuteration and are assigned to the vibration modes of the planar structure. The vibration modes of the simplified model H5O2 + .40W(OH)(4)(OH2) were calculated by the quantum chemical method with the B3LYP/6-31+G* level: The 3420 cm(-1) band is assigned to the overlap of the v(OwHw) of the v(7) mode with the A component of Fermi resonance among v(8)(v(OwHw)), v(3) (delta(OwHw2)), and v(11) (gamma(OwHw2)) with the same species, of which the first mode is broadened by the anharmonic coupling of O-w-H-w and O-a-H-w vibrations. The bands at 2720 and 1780 cm(-1) are probably assigned to separated BC components of the Fermi resonance. The 2030 cm(-1) band is assigned to the overlap of overtone and combination vibrations of the skeletal vibration modes of v(as)(P-O), v(as)(W=O), and v(as)(W-O-c-W) in the same way to that Of Cs3PW12O40.OH2O: CS3PW12P40.0H(2)O showed three bands at 2159, 2074, and 1988 cm(-1), and the band positions were not changed by the deuteration and agreed with those of overtone and combination vibrations of skeletal vibration modes of V-as(P-O), v(as)(W=O), and v(as)(W-Oc-W). The 1640 cm(-1) band is assigned to the Hw-O-H bending vibration mode.