Pentacene ultra thin film transistors were exposed to water and operated with a conventional silicon/silicon oxide bottom gate and an electrolyte top gate controlled by a working electrode. The transistors are highly sensible (mu V) to the electrochemical potential of the aqueous electrolyte. We show that dual gate operation permits the measurement of the double layer capacitance, C-DL = 14.6 mu F/cm(2). The device exhibits a fast (4.6 ms) and stable response, without bias stress as opposed to conventional bottom gate operations, when controlled with the electrolyte gate. These features make the device a promising candidate for potentiometric transducers required for non-invasive electrophysiology.