Textile dyeing processes are among the most environmentally unfriendly industrial processes, because they produce coloured wastewaters that are heavily polluted with dyes, textile auxiliaries and chemicals. The ultrafiltration (UF) method was studied as a wastewater pre-treatment technique for the decolourization of residual dye-bath effluents after dyeing cotton/polyamide blends using reactive and acid dyes. Ultrafiltration was performed using a poly-ether-sulfone membrane with a molecular weight cut-off of 10 kDa and a pore size of approximately 10 nm. The transmembrane pressure of the UF system was 250 kPa. Colour and COD removals were evaluated in dye-baths as the difference between initial and effluent values. The quality of wastewater was improved in such way that the effluent enables the efficiency of further biological wastewater treatment.