Passive film formation as a function of applied potential on tin in citrate buffer solution was studied by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. In order to avoid air oxidation and to minimise contamination of the samples, an electrochemical preparation and transfer system attached to the ultrahigh vacuum system was used. Quantitative evaluation of the electrochemical and XPS data showed the characteristic changes of the spectra in the prepassive and in the passive potential range. The prepassivation correlated with the clear presence of Sn-0, Sn2+ and Sn4+ species, while true passivation correlated with the presence of Sn4+ species only, as expected on thermodynamic grounds. Three different oxygen-containing species were found in both potential ranges : oxide, hydroxide and water. The hydroxyl oxygen peaks (oxygen atoms from tin-hydroxide species) were dominant in the O 1s spectra due to electrochemical conditions used (first scan) to produce samples for XPS examination.