The electrorotation method can be applied to characterize the passive electric properties of individual cells or particles. For this, their frequency-dependent speed of rotation is registered microscopically. Commonly, object rotation is induced in harmonic rotating fields which are generated in four electrode chambers. The rotation speed of the objects is proportional to the square of the field strength. In this study, we calculated the two-dimensional electric field distribution in electrorotation chips using the FEMLAB finite element program (Comsol, Sweden). For reasons of symmetry, a perfectly circular field is generated at the centers of the four electrode chambers. Nevertheless, the field strength is reduced with respect to the quotient of electrode voltage and distance for any electrode shape. Distant from the center, the field polarization is elliptical with an eccentricity increasing with the distance to the center. For optimizing the electrode shape, the deviations in torques for given distances from the center have been calculated. These deviations have been compared for various electrode shapes in order to find an optimal chip design. Another criterion was a large electrode distance in the corners of the electrode arrangement in order to avoid field hot spots and to minimize dielectrophoretic particle collection. (c) 2006 Elsevier Ltd. All rights reserved.