Membranes were prepared by reacting diamine mixtures with trimesoyl chloride to produce composite nanofiltration polyamide membranes. The flux behavior as well as rejection profiles of monovalent and divalent ions were elucidated pertaining to the 3,5-diaminobenzoic acid (BA) content in the copolymer membranes. The polyamide characteristic group was confirmed by using the ATR-FTIR method. The pore size (r(p)), effective thickness/porosity (Deltax/A(k)), and surface charge density (X) of the membrane were investigated using the Donnan steric pore flow model (DSPM). The membrane with the addition of 0.05% BA shows some improvement in terms of flux. However, at higher loading of BA, flux is reduced. The rejection of divalent ions (Mg2+, SO42-) is higher than that of monovalent ions (Na+, Cl-). It was found that the diamine content and reaction time play key roles in determining the permeate flux, membrane parameters, and membrane separation capabilities.