Carbonylation of methanol and dimethyl ether in various acidic ionic liquids (ILs) over catalysts containing rhodium complexes has been investigated. It has been shown that the presence of the H2PO (4) (-) anion in IL leads to a significant increase in the rate of carbonylation of methanol and dimethyl ether to methyl acetate and acetic acid. Ionic liquids and conditions under which a conversion of 92% with a 80% selectivity for methyl acetate is achieved have been selected. The catalytic system proposed can be reused without loss of activity.