Nanofiltration of a high-formula-weight dye, Cibacron F3G-A, FW=840, was investigated at low- to intermediate-crossflow velocity and pressure using a zirconium hydrous oxide-poly(acrylic acid) membrane. A sharp minimum in the rejection vs, pressure curves obtained at constant crossflow velocities provided a distinct determination of the pressure required to initiate dynamic membrane formation. These minima occurred at a pressure near 2 MPa and only at a crossflow velocity less than 0.4 m/s, indicating that a critical pressure is required for membrane formation while also implying that the concentration at the membrane surface is involved in the modification.