Bioinspired solar water oxidation systems are being developed in the context of renewable alternatives to fossil fuels. One possible design incorporates thin-film dye-sensitized nanoparticle photoanodes to capture and convert visible light to charge carriers. Because the operation of these systems requires rapid and efficient charge separation and storage, time-resolved terahertz spectroscopy (TRTS), an optical pump/THz probe technique, is ideally suited for studying them due to its femtosecond time resolution and high sensitivity to the conductivity of nanoparticulate semiconductors. Using TRTS, the efficiency and dynamics of electron injection from a series of tris-pentafluorophenyl porphyrin sensitizers into TiO2 and SnO2 nanoparticles was investigated. Electron injection from the dyes to TiO2 was only observed when the energy level of the dye excited state was higher than that of the conduction band. When injection was possible, it was completed within 500 fs after photoexcitation. With SnO2, electron injection was slower, and the rate depended strongly on the identity of the metal ion coordinated to the center of the porphyrin ring. Additionally, the injection kinetics were multiexponential, which suggests that electron injection occurs from multiple excited states of the porphyrin molecule.