Macroscopic properties of electrorheological (ER) fluids are often determined by the spatial arrangement of the suspended particles. We investigate the microstructure of polydisperse ER fluids at low concentrations in a DC electric field. Optical microscopy reveals that the particles with polydispersity in size or in both size and dielectric property are mixed in stable single particle width chains, showing different spatial arrangement. This qualitatively agrees with the theoretical results obtained from an energy approach based on the finite element analysis. Thus, it seems possible to achieve various microstructures and hence desired macroscopic properties of ER fluids, by choosing appropriate polydispersity. (c) 2006 Elsevier B.V. All rights reserved.