We have investigated the sodium-promoted oxidation of the Si(113) surface by using X-ray photoelectron spectroscopy (Mg K-alpha, 1253.6 eV). 0 1s, Si 2p and Na 1s core level spectra for the O-2/Na/Si(113) surface were measured with increasing oxygen exposure and at several annealing temperatures. It was observed that sodium enhanced oxygen adsorption by about three times. When the 1.4 monolayer Na/Si(113) surface was exposed to oxygen at 150 K, the sodium overlayer was oxidized but the silicon substrate was not. Subsequent annealing transferred oxygen atoms into the silicon substrate and desorbed sodium from the Si(113) surface. This oxidation process resulted in thin silicon dioxide films at about 670 K. The results an consistent with those from low index silicon surfaces, Si(100) and Si(111), and are interpreted within a previously suggested sponge model fur the role of all;ali metal in the: promoted oxidation.