Reactions are reported of early rare earth oxides, RE2O3 (RE = La, Pr, Nd) with Ta2O5 under hydrothermal conditions (650 °C, 1.5 kbar) in concentrated aqueous hydroxide (20-30 M KOH) as a mineralizer. Under various stoichiometries several members of two new structure types were isolated, Ln2TaO5(OH) (Ln = La, Pr) and Ln3Ta2O9(OH) (Ln = Pr, Nd). The analogous niobate La2NbO5(OH) was also obtained. Both structure types were characterized by single crystal X-ray diffraction and contain pentavalent tantalatum oxide octahedra and complex rare earth oxide frameworks. The Ln2TaO5(OH) structure type contains Ln-O8 and Ln-O9 building blocks and TaO6 octahedra in a 3-D framework. It contains a 3-D rare earth oxide framework formed by from zig-zag chains of rare earth oxides linking sheets of rare earth oxides. The tantalates form edge-shared Ta2O10 dimers occupying gaps in the rare earth oxide frameworks. The structure of Ln3Ta2O9(OH) contains two types of 2-D rare earth oxide slabs built of seven and eight coordinate rare earth metals. The tantalate units form 2-D slabs through a multiple corner-sharing scheme of TaO6 octahedra. The Ln3Ta2O9(OH) structure type has an interesting close structural relationship to the previously reported rare earth titanate La5Ti4O15(OH), which is discussed. The presence of hydroxide in the lattice is confirmed by IR spectroscopy and the H atom locations are assigned unambiguously using bond valence sums.