Two modifications of the new copper(l) iodide coordination polymer Cul(2-iodopyrazine) were obtained by the reaction of Cul and 2-iodopyrazine in acetonitrile. During this reaction, intensely yellow crystals of form I appear first which transform within several minutes to intensely red crystals of form 11 which is the thermodynamically most stable form at room temperature. In catena[Cul(2-iodopyrazine-N)] (form 1; a = 4.1830 (6) ; b = 10.814 (1) Angstrom; c = 17.961 (4) Angstrom; V = 812.5 (2) Angstrom(3); orthorhombic; P2(1)2(1)2(1); Z = 4), corrugated Cul double chains are found in which each copper atom is coordinated by one additional 2-iodopyrazine ligand. In poly[Cul(mu-2-iodopyrazine-N,N')] (form 11; a = 4,2679 (5) Angstrom; b = 13.942 (2) Angstrom; c = 13.017 (2) Angstrom; b = 92.64 (1)degrees; V = 773.76 (2) Angstrom(3); monoclinic; P2(1)/c, Z = 4), Cul single chains occur which are connected via mu-N,N coordination by the 2-iodopyrazine ligands to layers parallel to (010). The thermal behavior of both forms was investigated using simultaneous differential thermoanalysis, thermogravimetry, and mass spectrometry as well as differential scanning calorimetry and temperature resolved X-ray powder diffraction. On heating, both forms decompose to copper(l) iodide, and the decomposition temperature of form I is significantly lower than that of form II. From all experiments, there is no indication of a phase transition of one form into the other or for the formation of a phase with lower amine content.