Single crystals of layered niobium oxides, Rb(Mg0.34Nb1.66)O-5 and K(Fe0.43Nb1.57)O-5, were prepared at 1200 degrees C. The niobium oxides crystallized in the KTiNbO5-type structure and have the orthorhombic space group Pnma with Z = 4 and the lattice parameters are a = 6.571(1), b = 3.858(1), and c = 19.077(1) Angstrom for Rb(Mg0.34Nb1.66)O-5, and a = 6.533(1), b = 3.820(2), and c = 18.682(2) Angstrom for K(Fe0.43Nb1.57)O-5. The single-crystal structure refinements led to R = 0.032 and R-w = 0.031 for Rb(Mg0.34Nb1.66)O-5 and to R = 0.049 and R-w = 0.064 for K(Fe0.43Nb1.57)O-5. The niobium and iron/magnesium atoms an statistically distributed in a unit of doubled octahedra sharing edges. Alkaline ions in the interlayer were ion exchanged with protons to yield hydrated protonated compounds, H(M,Nb)(2)O-5. nH(2)O (M = Mg,Fe).