Adsorption of NO2 on Au(lll) surface, as well as bonding in AuNO2, has been investigated by means of ab initio and density functional calculations. For the AuNO2 complex, both MP2 and B3LYP calculations predict three isomers, namely trans-eta(1)-O nitrito, cis-eta(1)-O nitrito and eta(1)-N nitro isomers, among which the latter two isomers would have comparable stability and are candidates for the ground state of the complex. The eta(2)-O,O＇ nitrito isomer has also been considered and was found to be a transition state. For NO2 adsorption on Au(111), our B3LYP cluster model calculations suggest that the favorable mode would be a mu(2)-O,O＇ nitrito over a short bridge Au-Au pair site. The calculated vibrational frequencies of the NO2 adspecies are in reasonably good agreement with the experimental HREELS and IRAS spectra.