A new class of electrorheological (ER) material, rare earth (RE = Ce, Gd, Er and Y) complexes of phosphate cellulose, has been synthesized using microcrystalline cellulose, phosphoric acid, urea and RE(NO3)3 solutions as starting materials. The ER properties of suspensions of microcrystalline cellulose, phosphate cellulose [cellulose-P-ONH4] and the [cellulose(-P-O)(3)RE] complex particle materials in silicon oil have been investigated under DC electric field. The formation of rare earth complexes helps to decrease the shear stress and viscosity at zero electric field, and to enhance the ER effect of the materials. The shear stress (tau(E)) of the ER fluid (20% weight fraction) of a typical yttrium complex [cellulose(-P-O)(3)Y], the yttrium content of which is 0.04 mol/100 g, is 2.3 kPa at 4.2 kV/mm and 300 s(-1) with a tau(r) value (tau(r) = tau(E)/tau(0), where tau(0) is the shear stress at no electric field and 300 s(-1)) of 34.3, which is 18 times higher than that of pure microcrystalline cellulose suspensions. The improvement of dielectric loss tangent of the material, due solely to the formation of rare earth complexes, resulted in an enhancement in the ER effect of the material. In addition, the cellulose(-P-O)(3)RE materials possess better thermal stability, and their suspensions are more stable in the anti-sedimentation than that of the cellulose-P-ONH4 material. (c) 2006 Springer Science + Business Media, Inc.