Optical, structural and electrochromic properties of tantalum pentoxide-doped niobium pentoxide films prepared by sol-gel spin-coating method were studied. Niobium pentoxide sol was doped with tantalum pentoxide sol at 3, 6, 9, 12 and 15 vol% ratios. Thickness increment of the films per 3 % doping ratio was calculated around 4.2 nm. Analysis of refractive indices showed that refractive indices of the films decrease with increasing tantalum pentoxide doping ratio, which makes them suitable to be used in filters. Optical band gap values of the films were calculated around 3.2 eV. It was observed from X-ray fluorescence measurements that increasing tantalum pentoxide doping ratio results in an increase in the intensities of Ta L-alpha 1 and Ta L-beta 1 transition peaks of the films. An increase was observed in the charge density of the films with increasing doping ratio. Tantalum pentoxide-doped niobium pentoxide films showed good kinetics in CV measurements.