The effect of solute asymmetry on flux-limiting solute-membrane interactions was studied for the ultrafiltration of dilute aqueous solutions of surface-active and nonsurface-active solutes through commercially available cellulose acetate and polysalt complex membranes. Triton X-100, sodium dodecylbenzenesulfonate, alkyldimethylbenzylammonium chloride (Hyamine 3500), and Carbowax 600 were chosen as the model nonionic surfactant, anionic surfactant, cationic surfactant, and nonionic symmetrical solute respectively. Flux declines resulting from specific solute-membrane interactions were observed for Triton X-100 with cellulose acetate membranes and for the ionic surface-active agents with polysalt membranes. Although small flux declines were observed for the ionic surfactants with cellulose acetate membranes and for the nonionic surfactant with polysalt complex membranes, these small flux-limiting effects were within the range of those observed for the symmetrical and therefore nonsurface-active Carbowax 600. Retention of surface-active solutes was higher in all cases for higher solute concentrations. This abnormal behavior was not observed for Carbowax 600 with either membrane. In all cases, except for the cationic surfactant-polysalt membrane combination, the observed flux declines were completely reversed by flushing the membrane with distilled water. These results suggest, therefore, that while surfactants as a class of materials do not lead to significant flux-limiting membrane interactions, solutes that interact specifically, albeit reversibly, with a membrane surface may cause flux-limiting phenomena.