A series of compounds with the general formula Ln Ba2Cu2GaO7+δ (Ln = La-Gd, Y) and LnBa2Cu2 CoO7+δ (Ln = La-Yb, Y) have been synthesized and characterized. The phase formation is found to be sensitive to the size of the rare earth ion in both the series. For Ln = La-Eu, the Ga-substituted compounds are single phase and possess tetragonal Y-123 structure. In the Ln Ba2Cu2CoO7+δ series, single-phase formation is noted for all the Ln’s studied except Yb. An orthorhombic-to-tetragonal transition occurs across the rare earth series. For Ln = La-Er and Y, the phases are orthorhombic, and for Ln = Tm the phase is tetragonal. Oxygen content of the phases is found to be ∼7.0 for the LnBa2Cu2GaO7+δ series, whereas for the LnBa2Cu2CoO7+δ system, the value increases to ∼7.25 as expected. Electrical resistivity studies show that all the phases are semiconducting with activation energies in the range 0.03-0.28 eV. IR spectra and magnetic susceptibility of select compounds have been studied and discussed. © 1993 Academic Press, Inc.