Mixed phase nanocrystalline TiO2 powders (anatase–rutile) (Degussa P25) were prepared by hydrothermal modification. The preparation procedure took place at 200 °C for 1–10 days in an autoclave system with water as the solvent. Thus, different degrees of modification were achieved. TiO2 water modified nanocrystalline thin films were immobilized on glass substrates by applying a doctor-blade's deposition technique. A variety of spectroscopic [UV–vis reflectance, infra-red (IR), Raman, structural X-ray diffraction (XRD), N2 absorption (BET)] and microscopic [atomic force microscopy (AFM), scanning electron microscopy (SEM)] techniques were applied to characterize the modified films. A model textile industry pollutant (methyl orange) was used in order to evaluate the photocatalytic efficiency of the modified material. Our results show that the photocatalytic activity of the modified films is improved by a factor of 2 when we extend the hydrothermal treatment up from 1 to about 4 days in the autoclave system. Scratch tests revealed favorable interconnection of the titania nanoparticles as well as significantly higher adhesion to the glass substrate for the modified films, in comparison to the original P25 material.