Two types of carbonyl iron powders, (CIP's, BASF AG), the HS and HS-I (I = insulated, due a coating with phosphate), and two kinds of silica, one hydrophobic (Cab-O-Sil@ TS610) and other hydrophilic (Cab-O-Sil@ M5), were used to evaluate the influence of the surface treatment of the magnetic particle and the kind of fumed silica on the formulation of some magnetorheological suspensions (MRS). Oscillatory measurements at no field showed an evident difference between the silicas, but not a specific interaction with the phosphate coating on HSI. On the other hand, steady flow experiments also without magnetic field showed that the kind of silica and its specific interactions with the coating on iron powder drove the rheological behavior of the MRS on all region of the shear rate. Under magnetic field, the flow curves differences will be due to the iron particles and its magnetic properties, mainly on the region of higher shear rate. 187194Viota, J.L., De Vicente, J., Jdg, D., Delgado, A., Stabilization of magnetorheological suspensions by polyacrylic acid polymers (2005) Journal of Colloid and Interface Science, 284 (2), pp. 527-541Lim, S.T., Choi, H.J., Jhon, M.S., Magnetorheological characterization of carbonyl ironorganoclay suspensions (2005) IEEE Trans. on Magnetics, 41 (10), pp. 3745-3747De Vicente, J., Lopez-Lopez, M.T., Gonzalez-Caballero, F., Jdg, D., Rheological study of the stabilization of magnetizable colloidal suspensions by addition of silica nanoparticles (2003) J. Rheology, 47 (5), pp. 1093-1109Klingenberg, D.J., Rankin, P.J., Horvath, A.T., Magnetorheology in viscoplastic media (1999) Rheol. Acta, 38 (5), pp. 471-477Park, O.O., Park, J.H., Kwon, M.H., Rheological properties and stability of magnetorheological fluids using viscoelastic medium and nanoadditives (2001) Korean Journal of Chemical Engineering, 18 (5), pp. 580-585Wereley, N.M., Bidisperse MR fluids using Fe particles at nanometer and micron scale (2006) J. Intelligent Material Systems & Structures, 17, pp. 393-401Park, O.O., Chin, B.D., Park, J.H., Kwon, M.H., Rheological properties and dispersion stability of magnetorheological (MR) suspensions (2001) Rheol. Acta, 40 (3-21), pp. 1-219Johansson, E., Nyborg, L., XPS study of surface-active organic compounds on fine ferrous powder (2000) Surface and Interface Analysis, 30 (1), pp. 333-336Heim, L., Farshchi, M., Morgeneyer, M., Adhesion of carbonyl iron powder particles studied by atomic force microscopy (2005) Journal of Adhesion Science and Technology, 19 (3-5), pp. 199-213(1993) Cab-O-Sil Division, , Cabot Corporation Cab-O-Sil® Untreated fumed silica properties and functions, Tuscola-USAKhan, S.A., Zoeller, N.J., Dynamic rheological behavior of flocculated fumed silica suspensions (1993) J. Rheology, 37, pp. 1225-1235Sazonova, V.F., Kojemyak, M.A., Perlova, O.V., Adsorption of tributyl phosphate on silica gel (2001) Adsorption Science & Technology, 19 (21), pp. 1-217Ginder, J.M., Davis, L.C., Elie, L.D., Rheology of magnetorheological fluids: Models and measurements (1996) Int. J. Mod. Physics B, 10 (23-24), pp. 3293-3303See, H., Field dependence of the response of a magnetorheological suspension under steady shear flow and squeezing flow (2003) Rheologica Acta, 42, pp. 86-92