Abstract The ternary oxides, (La,Pr)2CuO4, (Gd,Tb)2CuO4, (La,Tb)2CuO4, (La,Ca)2CuO4 and (La,Ba)2CuO4, were prepared and their semiconducting properties were investigated. When the mean ionic radius of Ln3+ (\barr) is increased, the (La,Pr)2CuO4 compounds transform from semiconductors to metals at a critical \barr value of 1.05 Å. The metal-semiconductor transition which accompanies the shrinkage of the a axis, is explained as being due to the π-bond formation of the Cu–O bond. Compounds in the composition range from La1.3Tb0.7. CuO4 to La1.4Tb0.64CuO4 are peculiar to the (La,Tb)2CuO4 compounds. The crystal structure is the same as that of Ln2CuO4, but it is different from Ln2CuO4 as regards the c/a ratio and resistivity. The doping of 20 mol% CaO or 10 mol% BaO into La2CuO4 lowers the resistivity to tenth of that of La2CuO4.