A method of making acetic acid includes: (a) catalytically reacting methanol or a reactive derivative thereof with carbon monoxide in the presence of a homogeneous Group VIII metal catalyst and a methyl iodide promoter in a reactor vessel in a liquid reaction mixture including acetic acid, water, methyl acetate, methyl iodide and homogeneous catalyst, the reactor vessel being operated at a reactor pressure; (b) withdrawing reaction mixture from the reaction vessel and feeding the withdrawn reaction mixture along with additional carbon monoxide to a pre-flasher/post reactor vessel operated at a pressure below the reactor vessel pressure; (c) venting light ends in the pre-flasher vessel and concurrently consuming methyl acetate in the pre-flasher/post reactor vessel. Reaction conditions, residence time and composition are controlled in the pre-flasher/post reactor vessel such that a pre-flash mixture is enriched in acetic acid and diminished in methyl iodide and methyl acetate in the pre-flasher/post reactor vessel. From the pre-flasher/post reaction vessel the acetic acid enriched mixture is (d) withdrawn and fed to a flash vessel.