Acidic, chloroaluminate ionic liquids (IL's) formed from combining two moles of AlCl3 with 1 mol of 1-R-3-methyl-imidazolium-chloride (RMIM-Cl, R = ethyl, n-butyl, n-hexyl, n-octyl, n-dodecyl, or benzyl) were examined as conversion agents for toluene carbonylation at room temperature when HCl and CO were present in the gas phase at partial pressures of 3.04 and 8.16 atm, respectively. The solubility of HCl and CO into the ionic liquids was measured at room temperature in separate experiments to determine the effect, if any, of the cation structure upon gas solubility. Molecular modeling by semi-empirical methods (AM-1; PM3, and MNDO) were used to simulate the absorption of HCl into the ILs when R was changed. (c) 2005 Elsevier B.V. All rights reserved.