Phase transitions and the mobility of proton-containing groups in hydrogen zirconium phosphate HZr2(PO4)(3)center dot nH(2)O with the NASICON structure were studied by X-ray powder diffraction, H-1, P-31 NMR, IR spectroscopy and TG analysis. Heating HZr2(PO4)(3)center dot H2O above 420 K results in dehydration and in a rhombohedral-triclinic phase transition. Continued heating to about 490 K results in the thermal activation of cation disordering and phase transition of HZr2(PO4)(3) from triclinic to rhombohedral phase. Parameter "a" of HZr2(PO4)(3) lattice decreases during the heating. It is shown that oxonium ions in HZr2(PO4)(3)center dot H2O are characterized by high rotation and translation mobility. Rotation mobility of oxonium ions can be increased by the substitution of zirconium by yttriurn or mobium. (c) 2007 Elsevier Ltd. All rights reserved.