Unusual structure transformation in NH4[NbO(C2O4)2(H2O)2]∙nH2O was discovered and investigated by combination of techniques including NMR crystallography (variable temperature 1H, 2H, 13C and 93Nb solid-state NMR methods), X-Ray powder diffraction, simultaneous thermal analysis and DFT calculations. It has been revealed that the mixture of known oxalate phases NH4[NbO(C2O4)2(H2O)2]∙3H2O (Phase I) and NH4[NbO(C2O4)2(H2O)2]·2H2O (Phase II) treated at mild condition (temperature up to 100 °C in vacuo) induces an unknown crystal phase, catena-NH4[NbO2/2(C2O4)2(H2O)] (Phase III). The new orthorhombic phase has polymeric structure consisted of 7-coordinated niobium fragments [NbO2/2(C2O4)2(H2O)] connected by oxygen bridges (–Nb–O–Nb–) forming the infinite zigzag chains along the c→ axis. The bridging oxygen atoms are connected to Nb symmetrically with respect to the 2-fold axis. Each of them forms two identical bonds Nb-O of 1.892(6) Å length, indicating their origination from the condensation process. The niobium coordination polyhedron can be crossed by the plane passing through the Nb, (O)2/2, (H2O) atoms. The oxalate groups lie in the opposite sides from the plane. In the new phase, 93Nb has unusually small nuclear quadrupole coupling constant mainly due to the developed thermal mobility of the crystal lattice. Variable temperature 93Nb, 2H and 13C NMR experiments together with DFT calculations confirm that the new Phase III has proton and ligand mobility. This mobility of the lattice correlates with the fact that hydrogen bonds network stabilizing the structure of Phases I and II practically disappear in the new Phase III.