An attempt was made to introduce a new approach for evaluating mass transfer during drop formation via definition of a parameter related to the extent of the convective mixing within the growing drop. For this purpose it was assumed that the entrance of the dispersed flow into the growing drop from the nozzle is analogous to the entrance of the flow from a smaller channel to a larger one. This transfer mechanism has been dubbed the "flow expansion." A global time-dependent Reynolds number of growing drop (Re-gd) was defined based on the equivalent diameter of growing drop as a length scale and also on a velocity scale, which is obtained using this flow expansion assumption. The results show that (Re-gd) has an important role on the mass transfer coefficient. The results of the model for prediction of instantaneous mass transfer coefficients and total cumulative mass transfer demonstrated relatively good agreements with experimental data. In some cases, however, for large nozzle diameter and relatively low nozzle velocity, i.e. in cases with large nozzle time scale (T-N = R-N / U-N), the flow expansion model showed some shortcomings. Subsequently, a modification called the transient flow expansion model was introduced which could improve the results of the previous model for large T, cases. Comparison of the results of the transient flow expansion model with experimental data reported in the literature showed relatively good agreement for a wide range of operational conditions: nozzle diameter, nozzle velocity, final formation time and various liquid-liquid systems. The capabilities of the model were also evaluated in comparison with other well-known analytical (surface stretch and the fresh surface elements) and semi-empirical models, along with available experimental data. The comparison of the model with the previous incorporated convection/circulating models demonstrated a high potential for this new approach. (c) 2005 American Institute of Chemical Engineers.