Phosphine adducts of zinc(II) halides, ZnX2L2 (X = Cl, Br, I; L = PMe2Ph, PEt3, PBu3, PPh3, PCy3), prepared by the reaction of ZnX2 with two equivalents of corresponding phosphines, showed high activities for the coupling reaction of CO2 and ethylene oxide to produce ethylene carbonate. The catalytic activity of ZnX2L2 was hardly affected by the nucleophilicity of the phosphine ligands, with the exception of ZnX2(PCy3)2, but depended strongly on the nature of halide ligands. An active species containing ethoxyphosphonium ligands, Zn3Br6[μ-OCH2CH2-P(C6H5)3]3, was isolated and characterized by X-ray crystallography, elemental analysis, and NMR spectroscopy. The mechanistic pathway for the formation of active species and cyclic carbonates is discussed.