The particle size distribution and magnetic susceptibility of some commercial carbonyl iron powders (code names CC, CS, HQ, OX and SM) were measured. The particle size of the powders increases as follows: HQ < SM < CC ≈ OX < CS. The magnetic susceptibility increases in the order: HQ ≈ OX ≈ SM ≈ CC ≈ CS. Magnetorheological suspensions (MRS) with 66% w/w of iron were prepared and their rheological properties were evaluated at no field, 100, 200 and 300 Oe. The yield stress under 300 Oe measured with strain-stress curves increases in the order: HQ ≈ OX < SM < CC < CS, showing direct correlation with the susceptibility. The plastic viscosity without field increases in the order: CS < CC < OX < SM < HQ, an inverse correlation with particle size. These results show that the particle size and/or size distribution can be another important property of the powders, together with magnetic susceptibility on the formulation of improved MRS.137-8471478Barnes, H.A., The yield stress - a review or 'παντα ρει - Everything flows? (1999) J. Non-Newtonian Fluid Mech., 81 (1-2), pp. 133-178Carlson, J.D., Catanzarite, D.M., St. Clair, K.A., Commercial magneto-rheological fluid devices (1996) Int. J. Modern Physics B, 10 (23-24), pp. 2857-2865De Gans, B.J., Duin, N.J., Van Den Ende, D., Mellema, J., The influence of particle size on the magnetorheological properties of an inverse ferrofluid (2000) J. Chem. Phys., 113 (5), pp. 2032-2042Farris, R.J., Prediction of the viscosity of multimodal suspensions from unimodal viscosity data (1968) Trans. Soc. Rheol., 12 (2), pp. 281-301Foister, R.T., Magnetorheological fluids (1997), US Patent 5,667,715 - September 16Ginder, J.M., Davis, L.C., Shear stresses in magnetorheological fluids: Role of magnetic saturation (1994) Appl. Phys. Lett., 65 (26), pp. 3410-3412Lemaire, E., Meunier, A., Bossis, G., Liu, J., Felt, D., Bashtovoi, P., Matoussevitch, N., Influence of the particle size on the rheology of magnetorheological fluids (1995) J. Rheology, 39 (5), pp. 1011-1020Ota, M., Miyamoto, T., Optimum particle size distribution of an electrorheological fluid (1994) J. Appl. Phys., 76 (9), pp. 5528-5532Popplewell, J., Rosenweig, R.E., Magnetorheological fluid composites (1996) J. Phys. D: Appl. Phys., 29 (9), pp. 2297-2303Rabinow, J., The magnetic fluid clutch (1948) AIEE Trans., 67 (PART II), pp. 1308-1315Rabinow, J., Magnetic fluid torque and force transmitting device (1951), US Patent 2,575,360 - November 20Rosenweig, R.E., On magnetorheology and electrorheology as states of unsymmetric stress (1995) J. Rheology, 39 (1), pp. 179-192Saunders, F.L., Rheological properties of monodisperse latex systems - I. Concentration dependence of relative viscosity (1961) J. Colloid Sci., 16 (1), pp. 13-22Shih, Y.-H., Conrad, H., Influence of particle size on the dynamic strength of electrorheological fluids (1994) Int. J. Modern Physics B, 8 (20-21), pp. 2835-2853Shulman, Z.P., Kordonsky, V.I., Zaltsgender, E.A., Prokhorov, I.V., Khusid, B.M., Demchuk, S.A., Structure, physical properties and dynamics of magnetorheological suspensions (1986) Int. J. Multiphase Flow, 12 (6), pp. 935-955Weiss, K.D., Carlson, J.D., Nixon, D.A., Method and magnetorheological fluid formulations for increasing the output of a magnetorheological fluid (2000), US Patent 6,027,664 - February 22