H2NO is the prototype of aminoxyls, kinetically persistent free radicals. The potential dimerization and reaction modes of H2NO are examined. The dimer potential energy surface features a barely metastable O-O bound species and several locally bound dimeric structures. One of these, a rectangular or rhomboid O-N-O-N ring, is a characteristic structural feature of more stable aminoxyls in the solid state. Its electronic structure is related to other four-center six-electron systems. A general picture of the weak dimer binding is constructed for these and other H2NO dimers from a balance Of four-electron repulsions between NO pi electrons, and two-electron attractive interaction between the singly occupied pi* orbitals of the diradical. The most stable diradical structure is a surprisingly strongly hydrogen bonded dimer diradical. The barriers separating the other isomers from this global minimum are calculated, to be small.