As part of the study of interaction of the Ba 2 RCu 3O 6+z ( R=lanthanides and Y) superconductor with SrTiO 3 buffer, phase equilibria of the subsystem, R 2O 3–TiO 2–CuO ( R=Nd, Y, and Yb), have been investigated in air at 960 °C. While the phase relationships of the two phase diagrams with smaller R (Y and Yb) are similar, substantial differences were found in the Nd 2O 3–TiO 2–CuO system, partly due to different phase formation in the binary R 2O 3–TiO 2 and R 2O 3–CuO systems. R 2CuTiO 6 and R 2Cu 9Ti 12O 36 were the only ternary phases established in all the three diagrams. R 2Cu 9Ti 12O 36 belongs to the perovskite-related AC 3(B 4)O 12 family which is cubic Im3. Depending on the size of R 3+, R 2CuTiO 6 crystallizes in two crystal systems: Pnma ( R=La–Gd), and P6 3cm ( R=Dy–Lu). The structure and crystal chemistry of the Pnma series of R 2CuTiO 6 ( R=La, Nd, Sm, Eu, and Gd) are discussed in detail in this paper. Patterns for selected members of R 2CuTiO 6 have also been prepared and submitted for inclusion in the Powder Diffraction File (PDF). Phase diagram of the Y 2O 3–CuO–TiO 2 system prepared in air at 960 °C