Abstract An UHF-CI investigation of parts of the energy surface for the H 3O radical is reported. Several types of basis sets have been used and the CI expansion included all singly and doubly replaced configurations using an UHF determinant as the reference state. H 3O, constrained-to C 3v symmetry is in the best approximation found to be 20.5 kcal/mol less stable than H 2O + H. A small local barrier of 4.6 kcal/mol for dissociation is found on the UHF level of approximation. Correlation effects lower this barrier to 3.4 kcal/mol making the existence of a quasibound state with a measurable lifetime improbable. The height of the barrier was found to be very sensitive to the detailed form of the diffuse singly occupied orbital.