The photochemistry of N2O3 adsorbed on a Au(111) surface has been studied at < 100 K, N2O3 is prepared by a reaction of NO2 chemisorbed on a Au surface with gas-phase NO. The adsorption states of N2O3 are characterized by IR reflection-absorption spectroscopy. Adsorbed N2O3 is found to undergo rapid photodissociation to form chemisorbed NO2 and gas-phase NO under UV and visible illumination. The photodissociation cross section is determined to be (3.0 +/- 1.8) x 10(-17) cm(2) at 350 nm, which is much greater than the maximum absorption cross section of gas-phase N2O3 and one of the highest values ever observed for photochemical reactions on a metal surface. The spectral profile of the NO yield is significantly shifted to longer wavelengths as compared to the absorption spectrum of gas-phase N2O3. The significant red shift of N2O3 absorption in aromatic solvents suggests that electron transfer from the surface to N2O3 enhances the absorption cross section of adsorbed N2O3 at longer wavelengths.