Recent correlations of membrane polymer structures with desalting properties in hyperfiltration experiments have demonstrated the high desalting performance of polyimide membranes (1). Membranes were synthesized which were suitable for seawater desalination with a salt rejection of about 99.7% at 100 atm and at a feed solution of 0.6 M NaCl. The corresponding volume flux of dense membranes at 10 pm thickness was about 14 liters /m2d. These results act as a stimulus to achieve a further increase in volume flux without the decrease in salt rejection which occurs usually with more hydrophilic and thus more water-permeable hyperfiltration membranes. It was expected that the rigid structure of the heterocyclic polymer backbone should facilitate an increase of water transport through the membrane without coupled increase of salt transport. Methoxylated aromatic monomers were synthesized and applied for synthesis of improved polyimide and polyamide membranes. In addition, experiments were performed to achieve soluble polyimides for hyperfiltration membranes and to stabilize these polyimides against hydrolytic attack.