Nanocrystalline TiO2 electrodes were studied spectroelectrochemically by observing the simultaneous relaxation of the current and absorbance after applying a voltage step. The absorbance behaved differently in two time regimes: (1) ionic polarization in the oxide electrode, in which charged ions, such as Ti3+ sites and/or interstitial Ti4+ sites, move in response to the applied electric field, and ( 2) the diffusion of Li+ ions into the TiO2. These two behaviors were analyzed with equivalent circuit models. Li+ ions reduce the resistance of the TiO2 by similar to 90%, increase the capacitance by similar to 350%, and decrease the inductance by similar to 30%. Voltage cycling produces a buildup of intercalated Li+ ions, lessening the electrode's response to the potential step, and causing it to become a more efficient inductor. The potential distribution in the nanoparticles is described by using a dielectric model in which roughly half the applied potential is dropped across the interface with a Li+-ion-ontaining electrolyte.