Cross-flow ultrafiltration is an important industrial process, which is inherently limited by concentration polarization and fouling. The introduction of gas bubbles into the filtration feed improves flux. However, for design purposes, accurate models of the process are required. In the first part of this paper a mechanism of flux enhancement is proposed for gas-sparged hollow fiber membrane ultrafiltration. This mechanism is determined from previously published experimental and computational fluid dynamics studies of the capillary slug flow process, as well as from dimensional analysis of the process. A physicochemical. model for flux prediction is designed around the postulated enhancement mechanism. The flux-prediction model enables estimation of upper and lower bounds. The model comprises an approximate solution of the flow problem, assuming controlled gas bubble distribution, coupled with a one-dimensional, integral method boundary layer analysis and flux models. The boundary layer analysis is adapted to include the effect of wall suction. In the second part of the paper, this model was evaluated against experimental flux results. In certain cases, it was found that flux estimates for the equivalent one-phase filtration process was higher than the lower bound flux estimate for the two-phase process. This highlights that gas sparging can be detrimental to the hollow fiber membrane ultrafiltration process under certain sparging conditions. The predicted upper and lower bound flux values were correctly found to encapsulate experimental values and should therefore assist process design.