Methyl carboxylate esters can be used as additives to promote the zeolite catalysed formation of dimethly ether (DME) from methanol. By taking advantage of the well-known confinement effect in combination with further functionalisation the potency of the promoter can be markedly enhanced, giving significant increases in DME yield at promoter concentrations as low as 10 ppm relative to methanol. The promotion is readily reversible and promoter concentration can be used to fine tune the zeolite productivity to DME.