XPS, RBS, and NRA have been combined to study the mechanisms of Li-ion electrochemical intercalation in MoO3 thin films prepared by thermal oxidation of molybdenum metal. A direct anaerobic and anhydrous transfer was used from a glovebox (O-2 and H2O < 1 ppm), where the samples were electrochemically treated at selected potentials between 1.7 and 3.2 V versus Li+/Li, to the XPS analysis chamber. The thermal oxide film grown at T = 450 +/- 10 degrees C and P(O-2) = 100 +/- 10 mbar for t = 5 min consisted of a 20 nm thick MoO3 outer layer overlying a 13 nm thick inner layer of lower oxides (Mo2O5 and MoO2). Combined RBS/NRA analysis allowed the dosing of intercalated lithium and the determination of the composition of the lithiated phases. Li0.50MoO3, Li1.20MoO3, and Li0.21MoO3 were obtained after intercalation at 2.58 and 1.73 V and deintercalation at 3.2 V, respectively, showing that similar to 1.2 mol of Li can be initially intercalated in the potential range 1.7-3.2 V (capacity of 223 mA h/g), and similar to 0.2 mol of Li per mol of MoO3 is trapped in the oxide matrix after the initial stages of intercalation. The XP Mo3d core level spectra evidenced the reduction of Mo6+ ions to Mo5+ ions after intercalation at 2.58 V and further to Mo5+ and Mo4+ ions after intercalation at 1.73 V with resulting Mo6+/Mo5+/Mo4+ ratios of 53:47:00 at % and 37:39:24 at %, respectively. Reoxidation of molybdenum is observed after deintercalation but 40 at % Mo5+ subsist at 3.2 V due to the trapping of lithium strongly bonded to the oxide matrix. The Li1s core level (at E-B = 55.80 eV) is most intense at 1.73 V and does not vanish at 3.2 V. Broadening of the Mo3d core level peaks are assigned to the distortion of the oxide matrix. Changes of the electronic structure after intercalation result from the occupation of the Mo4d states (at E-B = 1.0 eV) originally empty in the pristine oxide. The XP C1s and O1s core level spectra show the irreversible formation of a solid electrolyte interphase (SEI) layer including lithium carbonate and Li-alkoxides.