Several electronic states of the peroxonickel(II) bis(isocyanide) molecule Ni(O-2)(CNH)(2) have been calculated using various ab initio methods in order to determine the ground state and the order of the low-lying excited states. Multireference configuration interaction calculations using natural orbitals predict a (1)A(1) ground state, in agreement with the observed diamagnetism for the analogous tert-butyl-substituted molecule. The first excited state was found to be the B-3(1) state, formed by excitation of an electron from a dioxygen pi(g) orbital, whose nodal plane corresponds to the Ni(O-2)(CNH)(2) molecular plane, to the virtual nickel 3d orbital which lies in the NiO2 plane. Natural orbital occupancies obtained from the multireference configuration interaction calculations show the (1)A(1) state to be formally Ni(II)-O-2(2-), although substantial occupation of the formally unoccupied nickel 3d orbital suggests that the contribution to the molecular wave function from the superoxo-like Ni(I)-O-2(-) description is strong. The superoxo description dominates the lowest excited state (B-3(1)), but here the singly occupied O-2 orbital is perpendicular to the NiO2 plane. In addition to these two states, three other low-lying triplet states were calculated, and the charge densities (rho and del(2) rho) of all five states were examined.