The motions of the planar diaquahydrogen ion H5O2+ in H3PW12O40. 6H(2)O are studied by H-1 nuclear magnetic resonance second moment (M-2) and spin-lattice relaxation time (T-1) measurements. The phase transitions were observed at 144 and 313 K by differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Comparison between the experimental and calculated M-2 values suggests that the pseudo-C-4 reorientation around the central H+ occurs at temperatures higher than 140 K and that H3O+-H2O behavior appears at temperatures higher than 315 K. Since the longitudinal magnetization relaxed nonexponentially, T-1 was also estimated theoretically by applying the hindered rotation treatment to magnetic relaxation for a five-interacting proton system. The enthalpy change (DeltaH) of 260 +/- 30 Jmol(-1) and the difference between the experimental and calculated T-1 minimum values show that at the order-disorder phase transition point of 144 K the potential energy well for the H5O2+ ion changes from a single potential well to unequal potential wells whose energy difference is 4-5 kJ mol(-1). The room temperature phase is dynamically disordered with the pseudo-C-4 reorientation of the H5O2+ ion.