A series of thiourea-based ionic liquids (TBILs) were developed as efficient catalysts for the synthesis of cyclic carbonates by the cycloaddition of CO2 with epoxides under metal-, cocatalyst- and solvent-free conditions. Different substituted groups including electron donating and withdrawing groups were introduced onto the para-position of the aromatic ring in cation of TBILs. The effect of the catalyst structures on the reaction activity was systematically investigated. Among the ionic liquids, TBIL3 with substituted OCH3 group showed the best performance with 99% propylene oxide conversion as well as 99% propylene carbonate selectivity under mild conditions (1.5 MPa, 130 A degrees C and 3 h) without any co-solvent. It was demonstrated that two secondary amine groups of the thiourea unit could dually activate the epoxides and CO2 and have a synergetic effect with the halide anion.