The synthesis, conformational preferences, hydrogen bonding behaviors, and membrane properties of new partially N-methylated random aromatic copolyamides were reported. These copolyamides were prepared by the low temperature polycondensations of isophthaloyl chloride with 3,5-diaminobenzoic acid, N,N'-dimethyl-4,4'-diaminodiphenyl ether (MDAE), and 4,4'-diaminodiphenyl ether. The incorporation of the N-methyl amide linkages into the polymer backbone decreased the contents of the cis conformation in the N-methyl amide linkages and suppressed the hydrogen bondings among the amide linkages. Furthermore, the surface hydrophilicity of the copolyamides evaluated by water contact angle measurements decreased with increasing the MDAE unit in the polymer backbone. These experimental results indicated that the suppression of the hydrogen bonding and the existence of the tertiary amide linkage in the cis conformation induced the loose packing of the polymer chains. As a result, the incorporation of the N-methyl amide linkage increased water flux and decreased salt rejection. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3453-3462