A molecular-level understanding of photo-induced electron transfer at semiconductor interfaces is important. We have recently found that the acceptor states (i.e. the conduction band) in nanocrystalline (anatase) TiO₂ can be widely tuned in aqueous and non-aqueous solution by controlling the concentration and the nature of cations present at the semiconductor-electrolyte interface. Therefore, the yield, dynamics, and mechanism for dye sensitization of anatase nanoparticles can be controlled in a systematic manner. Such systematic studies revealed conditions where ultrafast 'hot' interfacial electron injection is followed by rapid trapping on molecular acceptors. This presentation will focus on our most recent work in this area.