The photochemistry of NO2 adsorbed on an Au(111) surface has been investigated at < 120 K in an ultrahigh vacuum system. The adsorption states of adsorbates were characterized by thermal desorption spectroscopy and IR reflection absorption spectroscopy (IRAS). The adsorption of NO2 leads to formation of chemisorbed NO2, N2O4 (D-2h symmetry) in direct contact to the surface, and well-ordered N2O4 (D2h symmetry) physisorbed on these species without formation of other unstable N2O4 isomers. While the former two species undergo neither photodissociation nor photodesorption, physisorbed N2O4 is dissociated to NO2, NO, and adsorbed atomic oxygen under irradiation at lambda < 430 nm. IRAS showed that no intermediate species such as adsorbed NO2 or NO3 were formed during N2O4 photolysis. The photodissociation cross section of physisorbed N2O4 at 350 nm was found to be 5.6 x 10(-19) cm(2), which is close to the absorption cross section of gas-phase N2O4. Wavelength dependence of the cross section is very similar to the absorption spectrum of N2O4 adsorbed on LiF at 68 K. Physisorbed N2O4 photolysis is inhibited significantly when the Au surface is covered with a thin water ice film, suggesting that the photolysis is enhanced by metal substrate photoexcitation or electron transfer from the substrate to the adsorbates.