The effect of temperature, pressure, space velocity and feed-stream water content in the synthesis of dimethyl ether from methanol over a K-HZSM-5 catalyst was evaluated with respect to the activity, physicochemical properties and deactivation of the catalyst. Increasing the water content in the feed stream allowed to proceed the reaction over a wider temperature range. Despite a decrease in the methanol conversion at low temperatures (< 250 A degrees C), methanol conversions and dimethyl ether selectivities of more than 50 and 99%, respectively, were achieved. It was found that deactivation of the catalyst due to coke formation or dealumination occurred depending on the amount of water in the feed stream. Coke as covering the catalyst particle surface deactivates the catalyst. Aromatic compounds entrapped in the pores affect acidity, pore volume, surface area and also conversion. Although catalyst deactivation due to dealumination caused a 20% decrease in the methanol conversion, the stability of K-HZSM-5 is expected to be improved by the addition of a proper amount of water in the feed. MTD reaction over K-HZSM-5 at a reaction temperature of 340 A degrees C with various LHSV-pressure-water content conditions for 300 h on stream; (red) 10-10-0, (blue) 10-10-50, (black) 20-10-50, where the three numbers correspond LHSV (h(-1))-pressure (bar)-percentage of water. [GRAPHICS] .