The effect of added ethanol and (NH4)(2)SO4 on the nux decline index (FDI) of bovine serum albumin (BSA) and a fatty acid-poor derivative (BSA/FAP) was examined. Ternary phase diagrams of the two protein species indicated that the concentration polarization (CP) layer on the surface of a nonadsorbing 10 000 MWCO regenerated cellulose membrane had principally a packed bed structure up to 33 wt % ethanol and 21 wt % (NH4)(2)SO4. Intrinsic viscosity and turbidity analysis were conducted to determine the degree of intra- and interprotein interactions within this packed bed morphology. With BSA/FAP, the effects of these two interactions tended to counterbalance each other, so the FDI of this protein was not strongly influenced by solute addition. In contrast, the adsorption of fatty acids to BSA caused the protein to expand, producing a less rigid CP layer with a higher FDI. However, the addition of ethanol led to protein compression, reducing this effect. The presence of fatty acids also produced a more associated BSA in salt solution, which increased nux resistance. The results obtained for both proteins indicate that an FDI minimum is observed when a noninteraction hard sphere structure is present in the CP layer.