The geometrical and electronic structure of iodine nitrite, I-O-N=O, have been studied using DFT/B3LYP, MP2 and CCSD(T) methods. Topological analysis of the Electron Localization Function (ELF) performed at CCSD/TZVPP//CCSD(T)/TZVPP level reveals slightly polarized covalent N=O bond, the protocovalent N-O bond and mixed covalent-ionic I-O bond. The protocovalent N-O bond is described by two monosynaptic non-bonding basins, V(N) and V(O), with total basin populations of 1.1e in cis-IONO and 0.80e in trans-IONO, thus belonging to the charge-shift type. The I-O1 bond is essentially depleted with only 0.67e (cis) and 0.64e (trans). (C) 2011 Elsevier B.V. All rights reserved.