The disclosure relates to a process in which methanol is carbonylated in a reaction zone in the presence of a catalyst to obtain a reaction mixture (A) comprising acetic acid, hydrogen iodide, methyl iodide, water and the catalyst. At least a part of the reaction mixture (A) is withdrawn from the reaction zone. The withdrawn part of the reaction mixture (A) is introduced into a flash zone where it is brought into contact with an alkylimidazolium iodide to form a secondary mixture (B), and where the secondary mixture (B) is separated to obtain a vapor stream (BV) which comprises the acetic acid, water and methyl iodide, and a liquid stream (BL) which comprises the catalyst, the alkylimidazolium iodide and hydrogen iodide. The vapor stream (BV) is processed to purify the acetic acid, and the liquid stream (BL) is recycled to the reaction zone. The reaction mixture (A) is brought into contact with the alkylimidazolium iodide in the flash zone 1) by introducing to the flash zone separately from the withdrawn part of the reaction mixture (A) an extraneous alkylimidazolium iodide; or 2) by introducing to the flash zone separately from the withdrawn part of the reaction mixture (A) an alkylimidazole and forming the alkylimidazolium iodide in situ by reacting the alkylimidazole with the hydrogen iodide or the methyl iodide.