A mass transfer model based on an unsteady state mass balance over the concentration boundary layer, coupled with the back transport opposing the ultrafiltrate flux, has been developed in the present study. The model is capable of predicting permeate flux and rejection at different pressures, concentrations and stirrer speeds. This model first uses the experimental data of Bhattacharjee and Bhattacharya to integrate the governing partial differential equation and in the process, it calculates the back transport coefficient by a least squares fit. Once this coefficient along with other membrane and solute properties are known, permeate flux and rejection can be predicted at any desired operating conditions. Concentration profiles as a function of time for different experiments were also computed in order to analyze the effects of different operating parameters on the concentration boundary layer. The model predicts that the flux decline during ultrafiltration of PEG-6000 in a continuous stirred cell using a cellulose acetate membrane, occurs mainly due to the resistance offered by solute molecules during their transport back to the bulk. The effects of osmotic pressure and cake/gel formation are assumed to be negligible in this study.