The synthesis of 4-substituted 1,3-dioxolan-2-ones from the interaction between carbon dioxide and oxiranes in the presence of a catalytic system composed of tetraalkylammonium or phosphonium halide and calcium chloride has been investigated. It has been established that calcium chloride enhances the catalytic activity of the onium halides used. A mechanism describing the interaction between carbon dioxide and oxiranes has been suggested on the basis of the obtained experimental results. According to the assumed likely mechanism the catalytic system activates the oxirane and the opening of the oxirane ring is the rate-determining stage. (C) 2002 Elsevier Science B.V. All rights reserved.