We have developed a method for AC characterization of an organic thin-film transistor, by measuring the complex impedance between source/drain electrodes and the gate electrode at frequencies of 50 Hz to 20 kHz. Modeling the TFT as an RC transmission line, we find that the data cannot be explained by a model including only a real, frequency-independent sheet conductivity. Instead, we use the RC transmission line model to extract the frequency-dependent complex sheet conductivity of the semiconductor film. Particularly in the off-state, conductivity depends strongly on frequency and its imaginary component is greatly enhanced. This has significant consequences for practical electronic devices made from organic semiconductors.