A zinc-substituted sandwich-type polyoxometalate, Na-12[WZn3(H2O)(2)(ZnW9O34)(2)](.)46H(2)O, in conjunction with a Lewis base was found to be efficient for the cycloaddition of CO2 with epoxides to form cyclic carbonates. The catalyst system was tested for a few representative epoxides, including epichlorohydrin, propylene oxide and butylene oxide; the reactions were carried out at moderate CO2 pressure (0.4-1.0 MPa) in the temperature range 100-160degreesC. Using the present catalyst system, one could obtain near-quantitative conversion and high selectivity with or without organic solvents. The zinc-substituted polyoxometalate part of the catalyst system is heterogeneous, easy to separate after completion of reactions and reusable for many runs. (C) 2004 Elsevier B.V. All rights reserved.