The barrier for the radical isomerization CH 3 O→CH 2 OH is calculated by CBS-QB3 to be 29.7kcalmol −1 and lies higher (by 5.7kcalmol −1 ) than the dissociation limit CH 2 O+H. Hence, CH 3 O does not isomerize to the more stable CH 2 OH on its own. However, this barrier is reduced to 15.8kcalmol −1 when the CH 3 O radical is coordinated with protonated methanol (CH 3 O⋯HO(H)CH 3 + ) and the CH 3 O→CH 2 OH rearrangement can now take place within the complex. This rearrangement, which results in the hydrogen-bridged radical cation CH 2 O(H)⋯HO(H)CH 3 + can be viewed as an acid-catalyzed rearrangement. The ion CH 3 O⋯HO(H)CH 3 + represents the most stable form of the methanol dimer radical cation. The ion CH 2 O(H)⋯HO(H)CH 3 + can fragment directly to CH 3 OH 2 + +CH 2 OH or it can rearrange further to produce the hydrogen-bridged radical cation CH 2 O + (CH 3 )H⋯OH 2 , which is the dimethylether ylid cation solvated by water. This species can dissociate to its components or to CH 2 O⋯H + ⋯OH 2 +CH 3 via an S N 2 type reaction. Alternatively, CH 2 O + (CH 3 )H⋯OH 2 may undergo “proton-transport catalysis” to produce the complex ion CH 3 OCH 3 + ⋯OH 2 which then dissociates. Our calculations confirm for the most part recent experimental findings on the methanol dimer radical cation [Y.-P. Tu, J.L. Holmes, J. Am. Chem. Soc. 112 (2000) 3695] but they also provide a different mechanism for the key isomerization reaction observed in that study.