Acidic cesium salts of 12-tungstophosphoric acid promoted with rhodium, Rh/CsxH3-xPW12O40 (1.5 less than or equal to x less than or equal to 2 and Rh greater than or equal to 0.1 %), were shown to be novel and effective catalysts for halide-free carbonylation of dimethyl ether (DME) to methyl acetate. The catalysts were characterized by chemical analysis, nitrogen adsorption, XRD, SEM, TGA-DTA and IR spectroscopy. Their behavior may be explained by assuming that (1) activation of the C-O bond in the DME molecule and formation of a metal-alkyl bond occurs in the presence of the strong acid sites and (2) these strong acid sites act in conjunction with Rh carbonyl complexes, which are responsible for CO insertion and acetate formation.