A rheological model that accounts for the effect of the volume fraction, the particle size distribution (PSD), and the shear rate on the dispersion viscosity was developed. The model contains three parameters (k(1), k(2), and k(3)) that accounted for the interaction among particles. The model was validated in experiments in which the surfactant type and concentration, the volume fraction, the PSD, and the shear rate were widely varied. The parameters were observed to be independent of the volume fraction, the PSD, and the shear rate. Parameter k(1) increases with the surfactant content but was independent of the ratio anionic surfactant/nonionic surfactant. On the other hand, under the range of experimental conditions studied, k(2) and k(3) were not affected by the surfactant type and concentration.