Flux experiments were conducted to determine the effect of aqueous methanol, ethanol and acetonitrile solutions on the flow resistance behavior of 10 and 30 kDa molecular weight cutoff cellulose and polysulfone membranes. The resistance of the 10 kDa cellulose membrane increased as the solubility parameter of the aqueous solvent solution (delta(S)) approached that of the membrane (delta(P)). The opposite effect was observed with the 30 kDa cellulose membrane : a resistance minimum was observed when delta(S) congruent to S-P. Differences in flux behavior are believed to be a consequence of the 10 kDa membrane having a much mote anisotropic structure as compared to the 30 kDa membrane. Basically the same flux maxima and minima trends were observed with the Row of solvent solutions through 10 and 30 kDa polysulfone membranes. However, acetonitrile, because of its low hydrogen bonding capabilities, dramatically reduces the glass transition temperature of polysulfone. As a result, the presence of this solvent at high concentrations disrupted the integrity of the 10 kDa polysulfone membrane, leading to an over ten fold decrease in flow resistance.