The infrared spectra of nickel mononitrosyl complexes isolated in solid argon at low temperature have been reinvestigated. Concentration and photochemical studies demonstrate the existence of two isomeric forms of NiNO. Analyses of the Ni-58/Ni-60, N-14/N-14 and O-16/O-18 isotopic effects indicate that a first form has an end-on bent configuration, and a second form has a cyclic structure, in which the NO ligand is significantly more perturbed. Density functional calculations of the geometrical, electronic and vibrational properties of these two forms are also presented and compared to the experimental values. Isomerization energies have been estimated.