TiO2-based dye-sensitization cycle is one of the basic strategies for the development of solar energy applications. Although the power conversion efficiency (PCE) of dye-sensitized devices has been improved through constant attempts, the intrinsically fatal factor that leads to the complete failure of the PCE of TiO2-mediated dye-sensitized devices has not yet been determined. Here, by using isotopically labeled MAS-1H NMR, ATR-FTIR spectroscopy (separate H/D and 48Ti/49Ti experiments), and ESR, we revealed that the accumulative formation of Ti-H species on the TiO2 surface is the intrinsic cause of the PCE failure of TiO2-based dye-sensitization devices. Such a Ti-H species is generated from the reduction of hydrogen ions (mostly released from dye carboxyl groups or organic electrolyte) accompanied by electron injection on the surface of TiO2, which deteriorates the PCE mainly by reducing the electrical conductivity of the TiO2 (by a maximum of ∼80%) and the hydrophilic nature of the TiO2 surface (contact angle increased).