Discrete dinuclear and polymeric heterometallic copper(II)-lanthanide(III) complexes have been synthesized upon variation of pH and characterized by X-ray diffraction analysis. Reactions of the ligand Htza (tetrazole-1-acetic acid) with copper(II) and lanthanide(III) salts gave dinuclear [CuLn(tza)(4)(H2O)(5)Cl] complexes at the low pH of 3.5 and 2D heterometallic coordination polymers with high-nuclearity [{Cu-2(OH)(2)}(2){Cu(12)Ln(6)(mu(3)-OH)(24)(Cl)(1/2)(NO3)(1/2)-(tza)(12) (H2O)(18)}](NO3)(9).8H(2)O (Ln = Gd or Nd) at a higher pH of 6.6. The acidity of the reaction solution can cause drastic changes in the structures of the products. In the dinuclear complexes, each pair of adjacent dinuclear molecules is linked through hydrogen bonds and pi-pi stacking interactions, and the whole structure is a hydrogen-bonded three-dimensional cubic net. In the coordination polymers, the connecting nodes are [Cu(12)Ln(6)] units, which are interconnected by [Cu2O2] units into two-dimensional structures. Magnetic studies exhibit the existence of weak exchange interactions between the CuIII and Ln(III) ions bridged by carboxylate and hydroxy ligands.