The effect of film thickness on the semiconducting properties of passivating $TiO_2$ films has been studied by using AC impedance measurements. AC impedance measurements were carried out by superimposing an ac voltage of 5 mV amplitude on a bias of -0.8 to 1.4V (SCE) over a frequency range of 6.3 to $10^5$ Hz. The passivating $TiO_2$ films were formed galvanostatically with 4 mA/㎠ in $1NH_2SO_4$ solution at various potentials of 10, 20, and 30 V. The corresponding thicknesses of the films were estimated to be 46, 61, and 83nm, respectively. The Mott-Schottky plots revealed that the donor concentration Nd decreased from $3.9\times10^{19}$ to $1.16\times10^{19}/cm^3$ with increasing film thickness. Capacitance decreased with increasing film thickness, while resistance increased. The dependence of electrical properties on film thickness was attributed to an increasing crystallinity and stoichiometry of the passivating $TiO_2$ films. In addition, anodic current / potential characteristics for the passivating $TiO_2$ films with various thicknesses were examined to investigate the electron transfer reactions (ETRs) at the $TiO_2$ films / electrolyte interface. The interface is characterized by the ohmic characteristics. Anodic current decreased with increasing film thickness. It is suggested from the results that ETRs take place equienergetically from the electrolyte to the donor levels of the film and the amounts of the anodic current caused by ETRs are greatly affected by the thickness.