The successive application of Taylor's equation of relative viscosity to a toluene-glycerin dispersion system leads to a mathematical model expressing the function relationship between the relative viscosity of the dispersion, the volume concentration and the drop size distribution. Experimental results of toluene-glycerin dispersions from 5% to 25% of the volume of toluene were tested against the derived equation. The errors between the experimental viscosities and the values calculated by the proposed model ranged from 0.24% to 2.06%. The average error percentage was 1.109% and the maximum error percentage for the system was 2.06%. To validate the proposed model, for various dispersions, published data were applied to the derived model and compared with the calculated results. The errors between the experimental viscosities and the calculated values ranged from 0% to 3.86% depending on the dispersion materials. The same model was also applied to an emulsion system. The predicted viscosities of the emulsions using the model agreed reasonably well with the experimental data giving a maximum error of 4.1%.