An experimental investigation is described on the stability of magnetorheological fluids (MRFs) consisting of iron suspensions in silicone oil with a thixotropic agent (silica nanoparticles) as stabilizer. The theological properties were investigated using a commercial rheometer with a parallel-plate measuring cell. Several kinds of experiments were performed in steady-state, oscillatory, and transient regimes. The effects of the volume fraction of magnetic particles, the concentration of silica, magnetic flux density, B, and waiting time after preshear on the rheology of the MRFs were considered. Steady-state measurements demonstrated that our systems only display plastic behavior, for which a yield stress, sigma(y), is appreciable, for the highest iron concentrations and/or magnetic fields. The yield stress was found to be independent of the magnetic flux density when the concentration of silica particles was large enough ( > similar to 20 g/L). This is a manifestation of the entrapment of iron particles in the silica gel. The adhesion of silica on iron particles by acid-base (proton donation) reactions between both colloids in apolar media is also investigated as another mechanism that hinders the aggregation among iron particles under the external field action. For the same reasons, sigma(y) ceased to scale as B-2, or to increase with iron volume fraction, for such a threshold silica concentration. Oscillometric determinations were performed at a frequency of 1 Hz, and the complex viscosity was found to increase with B due to structure formation as a result of magnetic particle-particle interactions. In agreement with steady-state results, if the concentration of silica is sufficiently large, the complex viscosity reaches high values, but independent of magnetic flux density. Creep-recovery experiments are particularly sensitive to MRF stability, because the interplay between iron and silica concentrations, magnetic flux density, and waiting time after preshear, led to a broad range of behaviors, ranging from liquid-like to almost elastic solid. (C) 2003 The Society of Rheology.