Ultrafiltration (UF) and Diafiltration (DF) processes are typically performed at constant trans-membrane pressures (TMP) or with increasing TMP to maintain certain flux rate. For some proteins, such operations, however, can cause uncontrolled material buildup on the membrane wall, potentially leading to disruptive membrane fouling as well as product loss. Operation at constant, controlled wall concentration has been proposed by Van Reis et. al that can both help avoid these complications and achieve optimal performance of the process. Their outlined procedure for optimizing UF operation involves one single experimental run to generate sufficient data allowing determination of the parameters required for constant Cw operation, optimization of Cw, and accurate prediction of the respective process runtime. We tested this constant wall concentration (Cw) approach for three different classes of proteins: a monoclonal antibody from cell culture and two microbially-derived proteins -- a highly soluble one and a highly aggregating protein. We also gathered diffusional kinetic data for various proteins to explore the effect of protein concentration on its aggregation behavior. It was found that the constant Cw approach is effective for highly soluble proteins but encounters difficulty in predicting UF performance of proteins that have a strong tendency to aggregate in a concentration dependent manner. An alternative treatment for those proteins is being investigated.