In order to investigate the formation and the time evolution of Na oxidation states, a thin ayer 0 sodium was deposited on oxidized substrates and exposed to oxygen at 10(-9) Torr. In the Na 2p core-level spectrum, several chemically shifted components have been observed, denoting the presence of non-stoichiometric Na oxidation states, whose binding energy is substrate independent. Modification of these states, with oxygen exposure has been observed by detecting the increase in intensity of the corresponding shifted components and the consequent decrease of the clean features: the stationary configuration is reached in a time tau = 10(3) S.