Acetic acid is advantageously prepared from a starting material having a formic acid moiety and a methyl ester moiety by heating said starting material in the presence of (1) carbon monoxide, (2) at least one of (2-a) a metal accelerator selected from beryllium, a metal belonging to the group IIIA, IVB, VB, VIB or IIA of the Periodic Table, a lanthanide metal and an actinide metal and (2-b) an onium compound or a precursor for the onium compound, (3) a rhodium catalyst and (4) an iodine compound.