A single test allowing the stability determination of highly concentrated suspensions was elaborated by applying the technique of penetrometry. It consists of dropping a rod with known dimensions into the suspension with no initial speed. A mathematical model was developed and its validity tested using 75% coal slurries in water. The penetration of the rod is described by the equation: m d2x/dt2 + C(r)srho/2(dx/dt)2 + srhogx = mg. Approximated solutions of the rod equation were found and precise solutions proposed in particular cases. This technique reveals not only the stability of the suspension but also the different features (homogeneity, viscosity, presence of a sediment, etc.) of the dispersions. A good agreement was observed between the viscosity obtained by our technique and the experimental viscosity measured by a viscosimeter (Rotovisco).