Nonionic surfactants are widely used in industry, and large amounts of wastewater containing nonionic surfactants are produced each year. Nanofiltration (NF) is a possible option to purify these waters, reducing the overall water consumption and enhancing biological purification. However, the flux behavior of NF during purification of wastewaters containing nonionic surfactants is not well understood. NF tests were performed with both synthetic solutions and real wastewaters containing nonionic surfactants from carpet rinsing. When a membrane with a relatively high molecular weight cutoff (MWCO) was chosen, flux decreased to a level lower than that with most ultrafiltration membranes. When a low MWCO was chosen, flux either increased above pure water flux when a relatively hydrophobic membrane was chosen or decreased when a relatively hydrophilic membrane was chosen. NF thus seems feasible to reduce water usage in industrial processes involving nonionic surfactants when a proper membrane is selected. It appeared that flux is controlled by three mechanisms: first, the narrowing of membrane pores through adsorption of monomers when the MWCO is comparable or larger than the monomer size, causing flux decline; second, an improved wettability of the membrane surface through adsorption of monomers on hydrophobic groups, causing flux to increase above pure water flux; third, a decreased wettability through adsorption of monomers on hydrophilic groups, causing flux decline. The nonionic surfactant concentration, MWCO, and membrane's hydrophilicity determine which mechanism is dominant.