Application of the minimum diafiltration (DF) time solution for a monoclonal antibody resulted in a 20-h process time rather than the expected 12 h. Further investigation indicated high turbidity associated with a product solubility issue that caused a flux decline. As a result, the gel flux model and the associated minimum DF time were not predictive. Multiwell plate solubility screening confirmed that the protein passed through a region of low solubility during the ultrafiltration step. Multiple approaches to address this issue were considered and a new strategy involving variable volume diafiltration (VVDF) was developed. Process modeling and simulation were used to predict performance and to select a value of the DF ratio control parameter (buffer flow/permeate flow=0.65). Feasibility testing at the bench and pilot scales confirmed that the new strategy reduced solubility issues, fit within existing manufacturing tank volume and system area constraints, matched model predictions, and did not present significant implementation issues. Recommendations are made regarding the general value of this strategy, when it should be used, and how to implement it. (c) 2014 American Institute of Chemical Engineers Biotechnol. Prog., 30:646-655, 2014