Electric double layer transistors (EDLTs) have unique impedance properties. As the transport of charge in the semiconductor and of ions in the electrolyte involve different time scales, the gate-to-channel equivalent capacitance changes dramatically with frequency. An important feature is the coupling between the capacitance and the channel conductance in the frequency range of interest due to the relatively large time constant of the charging process. This paper presents a systematic study of these EDLT properties. An equivalent-circuit model is proposed that provides reasonable physical explanations and shows good agreement with the experimental results.