The hydrothermal reaction of Cr(NO3)(3), Ln(2)O(3), and iminodiacetate acid (H(2)IDA) in the molar ratio of 1:1:3 produced {[LnCr(IDA)(2)-(C2O4)]}(n)(Ln = Eu, 1; Sm, 2), which represent the first 3D 3d-4f interpenetrating coordination polymers. In the reaction, the H2IDA ligands partly decompose into oxalate anions (ox), which connect Ln(III) ions to form 1D {Ln(ox)}(n) chains. Each of the Cr-III ions is tridentate-coordinated by two IDA ligands, which act as tetradentate metalloligands to link {Ln(ox)}(n) chains to form 3D open networks. The two open networks interpenetrate each other to form nonporous products. 1 and 2 are thermally stable up to 327 and 360 degrees C, respectively. Both of complexes show normally paramagnetic behavior. The luminescent results imply that the energy transfers from Ln(III) to Cr-III are strong.