The dehydration of methanol and three deuterated isotopomers (CH3OD, CD3OH, and CD3OD) in M(+)(CH3-OH)(N), M = Cs or Na, cluster ions has been studied. The cluster ions are produced by collision of large neutral methanol clusters formed in a supersonic expansion with alkali metal ions. The energy imparted to the cluster by the collision and subsequent solvation is sufficient to induce reaction. The reaction was followed by comparing the intensities of M(+)(CH3OH)(N-2) (H2O) product peaks with the intensities of the patent M(+)(CH3OH)(N) peaks. A strong dependence of the extent of reaction on cluster size was observed. A minimum of twelve methanol molecules was necessary for the reaction to proceed in Cs+(CH3OH)(N) and a minimum of eight was necessary in Na+(CH3OH)(N). Isotopomers of methanol were used to confirm the identity of reaction products and to help suggest a reaction mechanism. When CH3OH and CD3OH clusters are produced, the exclusive products of dehydration are H2O and CH3OCH3 and CD3-OCD3, respectively. However, when CH3OD and CD3OD clusters are produced, the products are D2O and the corresponding symmetric ethers. We propose a possible reaction mechanism for the alkali ion catalyzed dehydration of methanol in the cluster ions.