A solid-state test platform has been designed and fabricated that allows characterization of candidate organic semiconductor materials used in organic field-effect transistors. Origins of electric-field modulation in these devices have been investigated. Using a modified four-point-probe technique, it has been found that in addition to the resistance of the organic semiconductor, the resistances of the source and drain contacts are also modulated by the gate electric field. A systematic experimental protocol has been outlined that allows the separation and study of contribution of modulated contact resistances and of the organic film resistance to the overall response. From these measurements the true carrier mobility of the organic semiconductor can be determined. (c) 2007 The Electrochemical Society.