The influence of contamination and agitation on single drop mass transfer was investigated, using a pilot rotating disc contactor. The chemical system of toluene-acetone-water, having a high interfacial tension, was used with the advantage of high accuracy and repeatability in using GC for analysis. It is ideally required to account for the extent of contaimnation in a quantitative manner and in both phases with a single parameter. For this purpose and for design applications, an improvement was made on a combined mass transfer model, to be consistent with experimental data without mathematical assumption. The influence of drop size was also considered. Based on the series of experimental data, a correlation was proposed for the coefficient of this model.