Three Al/Bi4Ti3O12/p-Si structures were fabricated with different interfacial layer thickness values (10, 25, and 53 nm) and admittance measurements of the structures were carried out between 1 kHz and 1 MHz in order to investigate the influence of interfacial layer thickness on dielectric properties of these structures. For the structure with thicker interfacial layer, higher dielectric constant (epsilon') and dielectric loss (epsilon '') values were obtained at low frequencies. At high frequencies, these parameters tend to be saturated and become almost constant. Loss tangent versus frequency plots exhibit a peak and magnitude of the peak weakens with increasing frequency and shifts toward high frequency region. The dispersion in epsilon' and epsilon '' with varying thickness at low frequencies was ascribed to interfacial polarization. In addition, obtained epsilon '' values indicated higher energy dissipation in the structure with thicker interfacial layer. Admittance data of the structures were also considered in terms of modulus formalism to interpret the relaxation processes in the structures. Moreover, dc electrical conductivity values were derived using frequency dependent ac electrical conductivity of the structures. (C) 2014 American Vacuum Society.