Li insertion into a thin anatase TiO2 film is studied for the first time in an ultra-high vacuum (UHV) environment. Using synchrotron radiation photoelectron spectroscopy, we demonstrate that the evaporated Li is inserted into the anatase and that the maximum amount that can be dissolved is 0.41 Li/Ti. Two most important properties are found in the UHV insertion process that can be directly related to electrochemical insertion: first, the Li 1s spectra suggest a two-phase regime and second, the insertion process is curtailed at the threshold for a phase transition that is not possible without the electrochemical driving force. (C) 2002 Elsevier Science B.V. All rights reserved.