Ion-sensitive properties of organic electrochemical transistors (OECT) based on Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) have been systematically studied for the first time. It has been found that the transfer curve (I(DS)-V(G)) of an OECT shifts to lower gate voltage horizontally with the increase of the concentration of cations, including H(+), K(+), Na(+), Ca(2+), and Al(3+), in the electrolyte. The gate electrode of the OECT plays an important role on its ion-sensitive properties. For devices with Ag/AgCl gate electrode, Nernstian relationships between the shift of the gate voltage and the concentrations of the cations have been obtained. For devices with metal gate electrodes, including Pt and Au, the ion sensitivity is higher than that given by the Nernst equation, which can be attributed to the interface between the metal gate and the electrolyte.