Various zinc halide-based ionic liquids were prepared from zinc halides and onium halides as catalysts for the synthesis of styrene carbonate (SC) from styrene oxide and carbon dioxide. The effects of the ionic liquid catalyst composition (types of onium cation and halide, onium cation/zinc ratio) and CO2 pressure on the reaction were investigated. The effectiveness of the onium cation as the active catalyst component was in the order of tetrabutylammonium > 1-butylpyridinium > 1-butyl-3-methylimidazolium >> choline. The type of halide atom originating from the zinc salt affected the catalytic activity of the ionic liquid more significantly than that of the one originating from the onium salt. The onium cation/zinc ratio also affected the SC yield, but its manner was different depending on the type of onium cation. Influence of CO2 pressure was not significant.