The oxonium radical (H3O) has been generated in its ground state by neutralizing a fast beam of ions in the near resonant electron transfer reaction H3O+ + K(g)→H3O* + K +. Analysis of neutral beam scattering profiles and collisionally reionized mass spectra indicate that the fully deuterated species (D 3O) can be formed in a distribution of dissociative and metastable states (τ > 0.6 μs). Thermalization of the precursor D 3O+, prior to electron transfer, is required for production of metastable D3O. Neither H3O nor D 2HO is observed in metastable states. These isotope effects support earlier theoretical predictions of a shallow local minimum on the oxonium potential surface. The ionization potential of D3O is calculated to be 4.3 ± 0.1 eV. Some spectroscopic implications for this radical are discussed. The oxonium monohydrates (H3O·H2O) are also observed to exist in metastable states for several H/D isotopic variants. The ionization potential of D3O·D2O is estimated to be ≥3.4 eV.