We report measurements of displacement current to study the transient effects of conducting channel formation and annihilation at organic semiconductor/dielectric interfaces. The device structure resembled a typical organic thin-film transistor with either source or drain electrode removed. However, the channel length was very long (1-6 mm) in order to increase the transit time and enhance the displacement current. The devices consisted of a gold electrode contacting a 30 nm thick pentacene thin film, thermally deposited on SiO2 dielectric, with a heavily doped p -type Si substrate serving as a bottom electrode. Electrical measurements were performed by measuring the displacement current running through the grounded gold contact while linearly sweeping the voltage bias applied to the bottom electrode. The processes of conducting channel formation and annihilation were observed as transients in the I-V characteristics. By integrating the displacement current with respect to time, the concentrations of carriers injected into, extracted from, and trapped inside the pentacene film during the voltage sweep cycle were determined. The results are analyzed in terms of a device model that includes the effects of traps.