A low-pressure membrane process was studied in order to treat aqueous solutions containing both dye and surfactant. The influence of various textile dyes (methyl orange, indigo carmine, amido black, titan yellow or direct black) on separation efficiency of surfactants (sodium dodecyl sulphate, SDS) was analyzed. Intersep Nadir membranes made of polyethersulfone, polysulfone, regenerated cellulose, cellulose acetate and polyamide were chosen. These membranes have different cut-off values (5, 10 and 30 kDa). Surfactant and dye concentrations in model solutions were equal to 100 g/m(3). The ultrafiltration process used pressures of 0.1 and 0.2 MPa. It was found that the SDS retention coefficient was affected not only by the kind of organic dye, but also by membrane material and membrane cut-off. For polyamide membranes the presence of organic dyes in separated solutions improved the SDS rejection, whereas for acetate cellulose and regenerated cellulose membranes, the opposite phenomenon was observed. The influence of dyes on SDS separation was almost negligible when polyethersulfone and polysulfone membranes were used. It seems that the mechanism governing SDS separation from solutions containing anionic dyes can be explained by the adsorption of surfactant monomers and dye particles in the pore interior of the membranes or by converting pre-micelles to micelles.