Anatase/rutile consituted TiO2 thin films were prepared by sputter-deposition, and the influence of post-annealing step with a narrow window at 200 °C, revealed a gaining factor of 5 in the H2 production. An in-depth analysis of the photocatalytic performance revealed the dominant role of intermediate states, rather than the hetero-crystalline nature and mesoscale structure. Structural, chemical and optical investigations based on scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, UV-visible spectroscopy and photoluminescence supported by ab-initio calculation, correlated the H2 production with the dual presence of OH − and Ti 3+ defects in the form of titanium interstitial atoms. In addition, steady-state photoluminescence measurements determined the chemically active role of ethanol, commonly used as a hole scavenger, into inducing deep hole traps upon dissociation on the surface. These results give new directions for the design of TiO2 based photocatalytic systems for light-driven H2 production through water splitting, guided by a detailed description of defects present on the electronic structure and their chemical identification.