XPS, RBS, and NRA have been combined to study the mechanisms of Li-ion electrochemical intercalation in MoO 3 thin films prepared by thermal oxidation of molybdenum metal. A direct anaerobic and anhydrous transfer was used from a glovebox (O 2 and H 2 O < 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 °C and P(O 2 ) ) 100 ( 10 mbar for t ) 5 min consisted of a 20 nm thick MoO 3 outer layer overlying a 13 nm thick inner layer of lower oxides (Mo 2 O 5 and MoO 2 ). Combined RBS/NRA analysis allowed the dosing of intercalated lithium and the determination of the composition of the lithiated phases. Li 0.50 MoO 3 , Li 1.20 MoO 3 , and Li 0.21 MoO 3 were obtained after intercalation at 2.58 and 1.73 V and deintercalation at 3.2 V, respectively, showing that ∼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 ∼0.2 mol of Li per mol of MoO 3 is trapped in the oxide matrix after the initial stages of intercalation. The XP Mo3d core level spectra evidenced the reduction of Mo 6+ ions to Mo 5+ ions after intercalation at 2.58 V and further to Mo 5+ and Mo 4+ ions after intercalation at 1.73 V with resulting Mo 6+ /Mo 5+ /Mo 4+ ratios of 53:47:00 at % and 37:39:24 at %, respectively. Reoxidation of molybdenum is observed after deintercalation but 40 at % Mo 5+ 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.