A series of ionic liquid-based zinc tetrahalide complexes were synthesized and their effects in the coupling reactions of ethylene oxide (EC) and carbon dioxide to produce ethylene carbonate (EC) were evaluated. The zinc tetrahalide complexes displayed high catalytic activities for the coupling reaction, but were found to cause a severe decomposition of EC during the vacuum-distillation of the product mixture. Among the various factors affecting the EC decomposition, the concentration of zinc tetrahalide species was most responsible for the decomposition: the lower the concentration, the lesser the decomposition. The addition of an imidazolium or ammonium halide was effective for enhancing the activity of the zinc tetrahalide complex while having only a minor effect on the decomposition rate. The optimum catalyst composition was discussed for producing EC feasibly with minimized product decomposition. (c) 2005 Elseviei B.V. All fights reserved.