Electrolyte-gated organic field:effect transistors (OFETs) hold promise for robust printed :electronics operating at low voltages. The polarization mechanism of thin solid electrolyte films, the gate insulator in such OFETs, is still unclear and. appears to limit the transient current characteristics of the transistors. Here, the polarization response of a thin proton membrane, a poly (styrenesulfonic acid) film, is controlled by varying the relative humidity. The formation of the conducting transistor channel follows the polarization of the polyelectrolyte, such that the drain transient current characteristics versus the time are rationalized by three different polarization mechanisms: the dipolar relaxation at high frequencies, the ionic relaxation (migration) at intermediate frequencies, and the electric double-layer formation at the polyelectrolyte interfaces at low frequencies. The electric double layers of polyelectrolyte capacitors are formed in similar to 1 mu s at humid conditions and an effective capacitance per area of 10 mu F cm(2) is obtained at 1 MHz, thus suggesting that this class of OFETs might operate at up to 1 MHz at 1V.