Potato starch phosphate (PSP) particles were synthesized via an esterification process of potato starch with a mixture of various concentrations of phosphoric acid and urea. The electrorheological (ER) characteristics of the synthesized PSP particles, suspended in a silicone oil, were examined. The PSP granule-based ER fluid exhibited typical ER behavior, possessing a nonvanishing yield stress under an applied electric field. Enhanced polarization and higher yield stresses were observed as the content of phosphate groups in the starch increases. (C) 2003 Wiley Periodicals, Inc.