A process for dehydrating methanol to dimethyl ether product in the presence of an aluminosilicate zeolite catalyst and a promoter selected from (i) aldehyde of formula R1CHO (Formula I) in which R1 is hydrogen, a C1-C11 alkyl group or a C3-C11 alkyl group in which 3 or more carbon atoms are joined to form a ring; or (ii) acetal derivative of an aldehyde of Formula I; and the molar ratio of promoter to methanol is maintained at 0.1 or less.