The present study is focused on exploring the effects of reaction conditions and ligand properties on the ultrafiltration performance during siRNA concentration. A series of charge-modified cellulose membranes were produced by chemical modification with sulfonic acid functionalities over different reaction times (8, 24 and 48 h), ligand concentrations (1, 2 and 4 M), and spacer arm lengths (varying the number of carbons between the sulfonic acid group and the surface between 2, 3 and 4 carbon atoms). Ultrafiltration experiments were performed over a range of feed siRNA concentrations in stirred ultrafiltration cells at constant / uniform transmembrane pressure (TMP). Membranes were characterized using X-ray Photoelectron Spectroscopy (to evaluate the Sulfur content) and using zeta potential measurements (to evaluate the effective membrane charge). The ligand charge density was higher at higher ligand concentrations and longer reaction times; however, the highest final siRNA concentration achieved during ultrafiltration was obtained using intermediate charging conditions (2 M ligand for 24 h). Membranes with higher degrees of modification showed better siRNA retention but suffered from lower initial fluxes, likely due to pore crowding / blockage effects. Increasing the spacer arm length from 2 to 4 carbons reduced the measured surface zeta potential from -13 to -10 mV, although the membrane with the longer spacer arm showed better performance during siRNA ultrafiltration. These results provide important insights into the factors controlling the performance and optimization of these novel charged ultrafiltration membranes for processing siRNA therapeutics.