Monolithic molecular separation membranes were prepared by utilizing concentration polarization-induced gelation of a particulate covalent network on a porous support. Polymerization of tetra-amine and di-isocyanate monomers produced a stable dispersion of nanoparticles of urea-bonded networks in organic solvent. As the sols were allowed to pass through the support, the critical gelation point was reached due to concentration polarization via accumulation of relatively larger network particles on the substrate surface, resulting in deposition of a nanoporous gel layer, which, upon drying, turned to a monolithic layer of a covalent network with interconnected pores estimated to be 2-10 nm wide. The resulting membrane filters off solutes larger than 3 nm. Facile control of the structure and thickness of the active layer along with the superior chemical and thermal stability are promising features for solvent-resistant nanofiltration membranes suitable for separation of organic, as well as aqueous, solution mixtures.