The vapor-liquid equilibrium (VLE) of toluene-ionic liquid (IL) from short- to long-chain imidazolium-based ILs (i.e., [C4MIM](+), [C8MIM](+), [C10MIM](+), and [C12MIM](+)) with various anions (i.e., [BF4], [PF6], and [Tf2N]) was first measured under wide concentration and temperature ranges. These thermodynamic data are of great significance for an in-depth understanding with respect to capturing condensable gases with ILs. The UNIFAC-Lei model was applied to describe the VLE and successfully extended from short- to long-chain imidazolium-based ILs. For the ILs with relatively short alkyl side-chains on cations (e.g., [C4MIM](+) and [C8MIM](+)), the vapor pressure depends on the types of both anions and cations, while for the ILs with long alkyl side chains on cations (e.g., [C10MIM](+) and [C12MIM](+)), the vapor pressure is mainly dependent on the type of cations. Moreover, the COSMO-RS model and quantum chemistry calculations were used together to provide microscopic insights into the effect of IL structures on the VLE of toluene-IL systems. (C) 2019 Elsevier Ltd. All rights reserved.