Highly filled suspensions have attracted much interest in the past decades since they may be required for specific applications involving for example thermal conductivity or mechanical strength. Their processing may need to be adapted since an increase in filler concentration generally comes along with an increased viscosity up to a critical content at which the viscosity diverges. In addition a “stable” and homogeneous dispersion is often a must. Understanding the mechanisms governing the stability of highly filled suspensions in the case of submicron zirconia powders in polyolefins is the aim of the present study. In particular, the processing method Ceramic Injection Molding (CIM) is one of the applications and requires both a high solid content and a stable, finely dispersed suspension even under high shear rates. Powder-matrix interactions are to be well understood to control both the rheology and the dispersion state. Powders agglomeration must be prohibited and increasing the solid contents already yields beneficial effects thanks to hydrodynamics forces. Steric stabilization using fatty acids is another effective method widely reported in literature. The beneficial effect upon the dispersion is generally supported by a decrease in the suspension viscosity and storage modulus. The present study discusses the dispersion mechanisms of stearic acid. For this purpose the relative viscosity and yield stress are more deeply studied. Both are measured using a plate-plate geometry of a stress-controlled rheometer. The viscosity is obtained at a given frequency. Yield stress values are extrapolated from stress-strain curves as the minimum stress required to bring the suspensions into motion. Contrary to what is generally claimed, adding stearic acid to a suspension can increase both the viscosity and yield stress. This has been observed at solid contents below a critical value. Discussion about the physical significance and practical consequences will be provided.