Abstract Clusters of magnesium and dimethyl ether (CH 3OCH 3) have been prepared in a supersonic expansion pick-up source. Solvated ions Mg +(dimethyl ether) n have been observed for n=2 and higher. Surprisingly, the single-photon ionisation process is very inefficient to form these clusters. Reaction products have been found to arise from an ion-molecule reaction where Mg + (3s 2 S) attacks the methyl oxygen bond in dimethyl ether. This reaction is made possible by addition of solvent molecules in the cluster, and the products are found with a maximum population when solvated by four molecules. This process, already observed in collisions of similar clusters and magnesium ions, is manifest here as an intracluster reaction.