To study plasmon-induced charge transfer mechanism between an excited gold nanoparticle (NP) and a TiO 2 NP, which can be applied to solar cell and photocatalyst technologies, ultrafast femtosecond visible-pump/infrared-probe transient absorption spectroscopy was utilized to explore charge separation and recombination dynamics in gold–TiO 2 NP systems. In this review, our recent works are summarized. TiO 2 NPs of different diameters were chosen as electron acceptors for a gold NP donor with a 10-nm diameter. Electron transfer from gold NPs to the conduction band of TiO 2 was observed by the transient absorption of electrons in the conduction band of TiO 2 at 3440nm after optical excitation of the surface plasmon band of gold NPs. By using a reference of Ru-complex dye sensitized TiO 2 film, the occurrence of ultrafast electron injection from gold NPs to TiO 2 NP film was clearly proved. It was found that electron injection was completed within 50fs and the electron injection yield reached 20–50% under 550nm excitation. The excitation wavelength dependence between 400 and 680nm suggested that there were two pathways for the injection: one was through direct electron-hole generation, and the other seemed to relate to enhanced electric field by plasmon. The charge recombination decay within 1.5ns was nonexponential and strongly dependent on the particle diameter of TiO 2 . Larger TiO 2 particles resulted in longer charge recombination times because of the longer diffusion length of electrons in TiO 2 particles. Also, two-photon absorption cross-section of the 10-nm gold NP at a near-infrared wavelength (1200nm) was estimated to be as large as 10 8 GM (1GM=10 −50 cm 4 sphoton −1 molecule −1 ).