Two new isostructural mercury(II) coordination polymers with two different counter-ions, [Hg(μ-bpen)Br2]n (1) and [Hg(μ-bpen)I2]n (2) {bpen=1,2-bis(4-pyridyl)ethylenediamine), have been synthesized and characterized by elemental analysis, IR spectroscopy and single crystal X-ray data diffraction. Bridging by the ligand bpen causes compounds 1 and 2 to form one-dimensional coordination polymers. The thermal stabilities of both compounds were studied by thermal gravimetric (TG) and differential thermal analyses (DTA). CH⋯Br and CH⋯I interactions in the solid state structure and their role in the self-assembly process are discussed.