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Hypervalent dimethyloxonium radical, (CH3)2O&z.sbnd;H⋅ (1), is formed by collisional electron transfer to protonated dimethyl ether in the gas phase and dissociates rapidly by cleavage of the O&z.sbnd;H and O&z.sbnd;C bonds. Ab initio and density functional theory calculations show that these dissociations originate from different electronic states of 1. The loss of H proceeds from the repulsive ground electronic (X) state of 1 and is 131 kJ mol−1 exothermic to form vibrationally excited (CH3)2O. The loss of methyl proceeds from the first excited electronic (A) state of 1 by crossing to the repulsive part of the X state potential energy surface, yielding vibrationally excited methanol. A substantial fraction of deuterated radicals, (CH3)2O&z.sbnd;D⋅ (1-OD), are metastable on the microsecond time scale. The metastable species result from the population of the B and higher excited states of 1 that are calculated to be bound along both O&z.sbnd;H and O&z.sbnd;C coordinates. The isotope effects on the metastability of 1 are explained by less efficient vibronic coupling between the bound B and dissociative A states in the deuterated radical. [Copyright &y& Elsevier]