We demonstrate, for the first time, work function fluctuations of a polyaniline film when used as the gate conductor of an insulated-gate field-effect transistor. The work function fluctuations induce drain current fluctuations, which are in excess of the channel noise of the transistor and the Nyquist noise of the polyaniline film. Using the fluctuation-dissipation theorem, it is determined that the fluctuations have a Lorentzian-like spectrum and are thermally activated with an activated energy of ca. 300 meV. The activation energy, as well as the corner frequency and magnitude of the fluctuations, depend on the applied electric field at the polyaniline–insulator interface. These results, along with coherence measurements, suggest that the fluctuations originate near the interface in the space-charge region of polyaniline. This technique provides kinetic and thermodynamic information about the gate conductor, at equilibrium, which cannot be extracted using other techniques that measure work function. Furthermore, by this technique it is possible to distinguish the fluctuations of the work function from the Nyquist noise. This approach should be generally applicable to any semiconducting material used as the gate conductor of a field-effect transistor.