By comparing the changes in π-π* absorption with the transconductance in PEO-LiCIO4 electrolyte-gated FETs, we have demonstrated that the high channel currents obtained at low gate voltages result from reversible electrochemical doping of the semiconducting polymer film. At low temperatures, the conductivity of the electrochemically doped poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT-C14, is nonlinear with a crossover from dσ(T)/dT > 0 to dσ(T)/dT ≈ 0 as a function of the source-drain voltage. High current densities, up to 10 6 A/cm2 at 4.2 K, can be sustained in the electrochemically doped PBTTT-C14 films. © 2007 American Chemical Society.