In dye-sensitized solar cells (DSCs), ultrafast electron injection from a photoexcited molecular sensitizer into the conduction band of a wide- bandgap semiconductor is key to efficient charge separation. According to the current understanding of DSCs functioning, charges are separated directly during this primary electron transfer process, yielding mobile hot conduction band electrons in the solid and holes localized on oxidized dye molecules at the surface. Basing on combined femtosecond transient absorption and time-resolved THz experimental data, we show here that intermediate interfacial charge transfer states (CTS) are rather formed upon ultrafast injection from photoexcited Ru(II)-polypyridyl sensitizers into mesoporous TiO2 films. Formation and dissociation of these CTS were found to strongly depend on their environment. This finding founds a new mechanism for charge separation in DSCs. It also provides a rationale for the effect of the electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions.