[en] The speciation of tetravalent technetium has been studied in HCl solutions ranging from 1.0 to 6.0 M. Tc(IV) exists in 6.0 M HCl as the well characterized TcCl₆^2- chloride complex. In 1.0 M HCl, the chloride ligands of TcCl₆^2- are slowly substituted by other ligands such as H₂O, OH^- or O^2-. So TcCl_mO_n(OH)_p(H₂O)_q^(4-m-2n-p)+ compounds may form with (m + n + p + q) = 6. Several speciation techniques such as UV-visible and Raman spectrometries, X-ray absorption spectroscopy and electrochemistry have enabled to get an insight into this problem. All the results have indicated two most likely species of Tc(IV) after aging for about ten days in 1.0 M HCl to be aquo-chlorocomplexes of technetium and not oxo-chorocomplexes: TcCl₅ . H₂O^- and TcCl₄ . 2H₂O. (orig.)