Ultraviolet or visible light induces chemical transformations via electronic excitation. Infrared radiation, corresponding to low-frequency molecular vibrations, can also bring about photochemical reactions by multi-photon absorption. Then, how low photon energies can we reach in photochemistry? In this proposed research, I will realize a novel concept of terahertz photochemistry: activating chemical reactions with intense terahertz (THz) light of extremely low photon energy (1 THz = 4.1 meV). Based on this novel approach, I will study aqueous proton transfer in external electric fields, the process underlying key bioenergetic phenomena as well as renewable energy technologies. Despite its obvious relevance, it has remained elusive how E-fields affect the known proton transfer mechanism in aqueous systems. Ultrashort THz pulses, on the one hand, will allow monitoring the THz photochemical proton transfer reactions in real time on the femtosecond time scale. On the other hand, THz light pulses will also serve as external E-fields, which bias the proton transfer reactions. Due to recent technological development, sub-picosecond strong field THz pulses on the order of hundreds of kV/cm with near single-cycle duration can be generated in the host laboratory, and the short pulses allow for ultrafast time resolution of ca 300 fs. Systematic studies varying field strengths and solvents will provide important insights into the local and external E-field effects on the proton transfer and the non-equilibrium solvation dynamics. The possibility of THz control over proton transport processes will also be examined by applying THz pulses at different delay time after photo-initiation.