Disubstituted trialkyloxonium ions R 1 OCH 2 O + (R 1 )CH 2 OR 2 (R 1 , R 2 = CH 3 or C 2 H 5 ) have been prepared by chemical ionization of dimethoxy- and diethoxymethane individually or as a mixture, and their fragmentation has been studied by means of metastable ion and collision-induced dissociations. It is found that when a methoxymethyl group is attached to the charged oxygen atom, the oxonium ions can fragment by C-O bond cleavage to generate a methoxymethyl cation/dialkoxymethane ion-neutral complex, in which methyl cation transfer occurs to expel neutral formaldehyde. When an ethoxymethyl group is connected with the central oxygen atom, a reaction channel involving loss of C 2 H 4 O is observed and found to be insensitive to collisions. This process is proposed to involve isomerization prior to fragmentation leading to methylated dialkoxymethanes coordinated with neutral acetaldehyde in ion-neutral complexes; these ion-neutral complexes are estimated to be 35 kJ mol - 1 more stable than the original oxonium ions.