The growth of the corrosion layer occurring when a sodium surface is exposed to air under room conditions was studied considering the ratio between O - K alpha and Na - K alpha characteristic peak intensities from the X-ray emission spectrum collected in a scanning electron microscope for different exposure times of metallic sodium to air. Comparison between experimental results and Monte Carlo simulations leads to the conclusion that Na oxidation in air proceeds in two stages: at relatively short times (up to 6.5 s) a Na-rich layer of composition Na2O is formed. At longer times the oxide layer evolves through reaction with oxygen to a composition that can be described by the stoichiometry Na2O2. While the first of these processes seems to be limited by the reaction of the oxygen molecule with sodium, the second one appears to be limited by oxygen diffusion in the growing layer.