The acidic, chloroaluminate ionic liquid (IL) derived from the combination of 1-ethyl3-methyl-]H-imidazolium chloride (EMIM-Cl) and aluminum chloride complex (AlCl3) in contact with HCl gas was modeled using the principles of equilibrium thermodynamics of chemically reacting systems as a means to understand better the reactivity of arene formylations in these IL's. The concentration of an HCl-chloroaluminate species, ClHAl2Cl7 that has been characterized as a super acidic Br phi nsted species, increases with increasing HCl partial pressure. Another chloroaluminate species Al2Cl7, which has been characterized as a strong Lewis acid, showed concentrations that decreased with increasing HCl gas phase partial pressures. The ultimate conversion of toluene to tolualdehyde could be correlated with the sum of the predicted molar amounts of ClHAl2Cl7 and Al2Cl7, by assuming that each ClHAl2Cl7 combined with 1 tolualdehyde and each Al2Cl7 sequestered a single tolualdehyde molecule. (c) 2005 American Institute of Chemical Engineers.