AbstractA new technique for the preparation of composite hollow fibers membranes and the optimization of the preparation procedure are described. This technique leads to a drastically reduced number of imperfections in the selective layer. A polyamide selective layer was prepared on the inner side of the fiber by interfacial polymerization. The well-known monomers m-phenylene diamine (MPD) and trimesoylchloride (TMC) have been applied for modification. The preparation procedure included the passage of an inert organic liquid between the monomer solutions. Without using the organic liquid, the formation of pin-holes in the film occurred and the selective properties were lost. The hollow fibers, tested under a pressure of 10 × 105 Pa (10 bar), had an average water permeability of 0.6 × 10−5 l (m−2 h−1 Pa−1) and a NaCl rejection of 99.5%. This is by far the highest rejection obtained with MPD/TMC based TFC-membranes, when tested with 10 × 105 Pa (10 bar) feed pressure. The results show that virtually defect free selective layers could be obtained using the developed technique. The performance of the membranes could be further improved by formic acid treatment to an average water permeability of 3 × 10−5 l (m−2 h−1 Pa−1) with rejections higher than 95%.