Optically passive thin films of CeO 2-TiO 2 mixed oxides with molar ratio of Ce/Ti of 0.05 were deposited by the spray pyrolysis technique (SPT) on a glass and fluorine-doped tin oxide (FTO)-coated glass substrates. Precursor solution containing cerium nitrate hexahydrate (Ce(NO 3) 2 •6H 2 O) and titanium tetraiso-propoxide (Ti(O i Pr) 4) having different volumetric proportions (0-5 vol% of Ti) in methanol were used. These films were characterized for structural, morphological, molecular, optical, electrochromic and colourimetric analysis. CeO 2-TiO 2 films deposited at 400 • C were found to be polycrystalline with cubic fluorite crystal structure. Transformation from polycrystalline to amorphous phase was observed with increasing TiO 2 content. The band centred at 539 cm −1 is assigned to Ce-O stretching vibration and the two medium intensity bands assigned to (Ti-O) and (Ti-O-Ti) stretching modes at 798 and 451 cm −1 , which confirms the mixed CeO 2 and TiO 2 phases. The band gap energy decreases (E g)f r o m 3.45 eV for pristine CeO 2 to 2.98-3.09 eV for CeO 2-TiO 2 films. The ion storage capacity (ISC) of CeO 2-TiO 2 thin film with 3 vol% Ti (Ce-Ti 3 sample) was found to be 26 mC cm −2 and electrochemical stability up to 30,000 cycles in 0.5 M LiClO 4-PC electrolyte. The optically passive behaviour of CeO 2-TiO 2 thin film is confirmed by its negligible transmission modulation (T ∼ 2.5%) upon Li + ion insertion/extraction, irrespective of the extent of Li + ion intercalation. The optical modulation of sputter deposited electrochromic WO 3 thin film was found to be enhanced from 56 to 61% with rapid increase in colouration efficiency (CE) from 42 to 231 cm 2 C −1 when CeO 2-TiO 2 is coupled as a counter electrode with WO 3 in an electrochromic device (ECD). On reduction of WO 3 thin film with CeO 2-TiO 2 as counter electrode, the CIELAB 1931 2 • colour space coordinates show the transition from colourless to the deep blue state (L * = 88.07, a * =− 2.37, b * = 24.59 and L * = 40.32, a * =− 1.16, b * =− 5.65) with steady decrease in relative lightness. Yxy and L * a * b * coordinates signify CeO 2-TiO 2 films and it also exhibits the application as counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion/extraction. Keywords. CeO 2-TiO 2 thin films; WO 3 thin film; counter electrode; colour analysis; electrochromism.