Co-feeding water with syngas in the direct synthesis of dimethyl ether has been studied in order to reduce the deactivation by coke deposition on the CuO-ZnO-Al2O3 metallic function in the CuO-ZnO-Al2O3/gamma-Al2O3 bifunctional catalyst. The runs (synthesis of methanol and synthesis of DME) have been carried out in an isothermal fixed bed reactor under conditions avoiding catalyst deactivation due to the sintering of the metallic function (<325 degrees C) and the deactivation of the acid function (with an excess of gamma-Al2O3 in the catalyst). The results of long-term experiments evidence that water in the reaction medium attenuates coke deposition during the methanol synthesis step, which is the limiting one. Water is understood to partially inhibit the formation of coke precursor methoxy ions. Furthermore, the formation of paraffins is attenuated by co-feeding water. (C) 2011 Elsevier B.V. All rights reserved.