The unique optical properties of porous anodic titanium oxide (ATO) make it a promising material for solar energy conversion, sensorics, and opto-electronics. The optical path length and effective refractive index (neff) of ATO can be tuned by chemical etching of pore walls. However, precise control of these optical parameters is still challenging due to the lack of data on the effect of pore widening time on the neff. Here, a detailed study of the influence of anodizing voltage and pore widening time on the neff of the ATO films was performed. Analysis of reflectance spectra of ATO synthesized at 35 – 50 V shows that pore widening in 3 wt. % H2O2 aqueous solution allows one to control the neff at values ranging from 1.54 to 1.84. The data required for the prediction of the thickness, neff, and optical path length of the ATO films from anodizing and etching conditions are obtained.