Thin-layer carbon/molecule/TiO2/Au electronic junctions with 1.7-2.1 nm thick molecular layers exhibit voltage-induced conductance switching, which may be repeated for at least hundreds of read/set/read/erase cycles. A fluorene(Fl)/TiO2 junction can be switched to a higher conductance "set" state by a positive voltage pulse and brought back to the lower conductance "erased" state by a negative pulse. Similar conductance changes occurred following exposure to H-2 or UV radiation, and the conductance change is completely inhibited in a dry atmosphere. The bias-induced conductance switching of TiO2 junctions is consistent with electrochemical reduction of Ti-IV oxide to the much more conductive Ti-III oxide, analogous to a solid-state redox reaction. The observations are consistent with reduction of hydroxylated TiO2 sites to a much more conductive Ti-III oxide by the electrons injected into the TiO2 during a positive voltage pulse. If Fl is replaced by aminodecane (C10H21N) or nitroazobenzene, the conductance switching is modified slightly, consistent with TiO2 being the active switching component. The results bear directly on the origin of the hysteresis observed in TiO2-based junctions and also on their suitability as examples of recently reported "memristors" [J. J. Yang, M. D. Pickett, X. Li, D. Ohlberg, D. Stewart, and R. S. Williams, Nat. Nanotechnol., 3, 429 (2008)].