Electrical test structures have been designed to enable the characterisation of interdigitated electrode structures in conductive solutions, as used in dielectrophoresis. Test masks have been fabricated to explore the impact of array size, finger separation and solution conductivity on the applied electric field. A circuit model based on a distributed RC network is proposed and evaluated, demonstrating close agreement with actual impedance measurements. This provides the capability to readily predict the voltage and phase along the length of electrodes used for dielectrophoresis and other applications.