Incorporation of Mo into pure WO3 thin films is of interest for electrochromic devices, as it improves colour neutrality in the dark state. Existing literature still lacks reliable quantitative data on the complex dielectric function of such coatings. In this study, we deposited WO3 and MoxW1−xO3 thin films by magnetron sputtering and subsequently characterised them by X-ray photoelectron spectrometry (XPS), grazing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM). In vacuo-lithiation was performed to incorporate lithium inside the thin films. The lithiated films were evaluated with variable-angle spectroscopic ellipsometry (VASE) from 340 to 990 nm and with transmission measurements from 330 to 2100 nm. The ellipsometric data were analysed with a straightforward model composed of the sum of Tauc-Lorentz and Lorentz oscillators dispersion laws. One Lorentz oscillator positioned around 1.3 eV is associated with the reduction of W6+ to W5+ by lithium incorporation. One Lorentz oscillator positioned around 2.3 eV is associated with additional states in the band-gap due to the presence of Mo in the lithiated film. The proposed model allows a better comprehension of the involved electronic transitions and allows to determine the refractive index and extinction coefficient of the materials precisely.