The oxidation and reduction processes of both polycrystalline tin and Sn(100) single crystals are investigated in borate buffer solution of pH 7.5 using cyclic voltammetry and in-situ characterizations such as Raman scattering, potential modulated reflectance spectroscopy (PMRS), and electrochemical scanning tunneling microscopy (EC−STM). Cross correlation of data yields new insights into the composition of the tin oxide film and its electronic properties as a function of oxidation conditions. We have unambiguously characterized the formation of at least three different tin oxides defined by the applied oxidation potential range with differentiated chemical composition and electronic properties. The first two oxidation peaks correspond to the formation of a tin hydroxide clearly detected by PMRSthat we propose to be a mixed Sn(II) oxy-hydroxideleading, at intermediate oxidation potentials, to the formation of a fluorescent oxide that we propose to be a Sn(IV) oxide with deep states in the middle of the semiconductor band gap, the doping concentration depending on both the applied potential and the oxidation time. At far anodic potentials, a hydrated and nanocrystalline tin (IV) oxide is also unambiguously characterized.