To investigate the possibility of applying the miscibility switch phenomenon to perform Friedel-Crafts acylation reaction, the high-pressure phase behavior of the ternary system containing acetylferrocene, the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) and carbon dioxide (CO2) is studied experimentally. Acetylferrocene is the product of the acylation reaction of ferrocene and has applications as an intermediate in the production of functional groups, in polymers, as combustion catalysts for propellants, and in medical chemistry. The experiments are performed using a synthetic method in the Cailletet apparatus within a pressure range of 0.25 MPa up to 10 MPa and in a temperature range of 278 K up to 368 K. Five different concentrations of CO2 [(10, 20, 31, 40, and 50) mol % of CO2] were investigated. In a Cailletet apparatus, temperature (or pressure) is kept constant at preset values, and the pressure (or the temperature) is varied until a phase change is visually observed for a sample with a constant overall composition. Changing the preset temperature (or pressure) allows measuring another point of the same sample. The solute effect on the phase behavior is studied by comparing the experimental results of the binary system [bmim][Tf2N] + CO2, which were collected previously, with those of the ternary system ferrocene + [bmim][Tf2N] + CO2. It is shown that the addition of an acetyl group to the ferrocene molecule dramatically changes the phase behavior of the binary system. The homogeneous liquid phase region is also determined experimentally, for which data are used to study the acylation in this work. This study indicates that performing the acylation reaction of ferrocene to acetylferrocene in the presence of [bmim][Tf2N] and CO2 in the occurrence of a homogeneous liquid phase is feasible.