Membrane filters are made from a wide variety of polymeric materials, in a wide range of pore sizes, and are used in large numbers (several hundred million per year) to filter particulates from many types of solutions. However, during filtration, organic solutes are adsorbed by these membranes, often quite extensively. Thus, the composition of the filtrate can be very different from that of the original solution. A systematic study has been initiated to quantify and explain the adsorption of organic solutes during membrane filtration as affected by membrane type (material, pore size, presence/absence of wetting agents), solute type, pH, ionic strength, and filtration flow rate. Our first studies have employed a dye (toluidine blue) as the dissolved organic solute, and 11 membrane filters made of mixed cellulose esters, polyvinylidene difluoride, polytetrafluoroethylene, and polycarbonate. The results of these studies, which are both equilibrium and kinetic in nature, are presented. These results indicate how adsorption effects may be minimized.