The effect of a cationic polyelectrolyte (PE) on the dye and surfactant removal from aqueous mixtures by low-pressure membrane process was assessed. Flat ultrafiltration (UF) membranes made of regenerated cellulose and polyethersulfone, and characterized by different cut-off values (5, 10, and 30 kDa) were used in the experiments. The UF process involved a pressure of 0.1 and 0.2 MPa. Permeation measurements were carried out with aqueous solutions of different anionic dyes (methyl orange, indigo carmine, amido black, and direct black) and dye solutions containing an anionic surfactant - sodium dodecyl sulfate (SDS), and/or a cationic PE. Dye, surfactant, and PE concentrations in the experimental solutions amounted to 100, 100, and 0.5 g/m(3), respectively. It was found that when a cationic PE was present in the dye-surfactant mixture, this was concomitant with a decrease in membrane permeability and an increase in dye and surfactant retention. This relationship was more pronounced for polyetersulfone membranes than for membranes made of regenerated cellulose, as well as for membranes of high cut-off values.